Hidden Gems

[Mostlynotgay]

Noxious Revival - A recursion spell or way to postpone an opponents reanimation. For 2 life or one green.

Head Games - Can be quite funny. Wait for someone to tutor, cast this then replace his hand with x lands or whatever screws them up.

I ran, and still run, noxious revival in my lantern deck. It lets you dig for stuff and put it back. It’s also amazing to screw with snapcaster players.

[WWolfe]

Noxious Revival - A recursion spell or way to postpone an opponents reanimation. For 2 life or one green.

Head Games - Can be quite funny. Wait for someone to tutor, cast this then replace his hand with x lands or whatever screws them up.

I ran, and still run, noxious revival in my lantern deck. It lets you dig for stuff and put it back. It’s also amazing to screw with snapcaster players.

It's an amazing card!

[Mnemosyne]

I have a few for Standard 

Herald of Anguish - Still a great card that isn't seeing play despite some great improvise decks. I can potentially use treasure tokens/servos to pay for its cost.

Metallic Rebuke - Another improvise card, like Mana Leak, can be as great when using 0 cost artefacts to tap.

Gleaming Barrier/ Servo Schematic - 0/4 for which creates a treasure on death is great with cards like Costly Plunder, same goes for Servo Schematic. The payback is there.

Perilous Voyage - 1U - return non-land permanent and scry 2 (if) spell =< 2 is a powerful card and underestimated card!

Liliana's Defeat - a sideboard card to be sure but a 1 mana sorcery to destroy ANY black creature/planeswalker is an awesome utility card! (Not to mention if you are playing against Liliana, Death's Majesty)

Thrashing Brontodon - a 3 mana 3/4 which can be sacked to destroy an enchantment/artefact isn't something to disregard, especially in a sideboard. Better than a Felidar Cub, as it can easily snub an Abrade/ Magma Spray

Wayward Swordtooth - As I've posted in my Top Rivals Cards, this was one of the ones I rated... 5/5 that can't attack for 3 mana but can still crew and act as Mina and Denn, Wildborn for land ramp, is brilliant!

Renegade Wheelsmith - 3 mana and it works well with Mardu Vehicles and in my opinion is a vastly better card than Ahn Crop Crasher. 3 mana and doesn't need to exert to utilise the same ability. The downside is that it doesn't have haste, but for a 3/2 that can crew, what's better?

Cultivator's Caravan - Another overlooked vehicle. Even though Abrade is still a mainboard card, this card can be a real asset. 3 mana for a 5/5 that can be used as mana ramp, and can't be targeted by Essence Scatter.

Kinjalli's Sunwing - A Thalia, Heretic Cathar but more resilient and has evasion. While it can't tap down non-basic lands it is such a value card!

Finally

Lathnu Hellion - A 4/4 with Haste for 3 mana? Why isn't this played, more often?!? I played it in a PPTQ and came 1st. You can even keep it alive by bouncing Greenbelt Rampager. They have to respond to it immediately, not many creatures turn 3 can withstand a 4/4, and the only removal that can effectively destroy it instant speed is Harnessed Lightning +1 Energy on the same turn. Unlikely you'll have a revolt trigger for Fatal Push, just saying!

[WWolfe]

That's some pretty good ones.

Restoration Specialist & Salvage Scout are pretty good recursion for cheap.

[Soren841]

Chain of Vapor is a secret MVP for storm in EDH

[WWolfe]

Beseech the Queen - may be a little situational, but better than Diabolic Tutor in almost every instance.

Chain Reaction - basically a red board wipe.

Skullwinder - essentially another Eternal Witness but it also gives an opponent a card back from their GY as well. Makes for some really interesting politics if you're EDH group is big into that.

[Varatius]

vedalken orrery

[Judaspriester]

Siren's Call - Force your opponent to attack with all creatures for U. All creatures that he hasn't played that turn and doesnt attack (e.g. tap creatures mana elves) get destroyed at end of turn

Maddening Imp - does the same as sirens call. BUT a) as tap effect, and b) he ignores summoning sickness. your opponent just played his commander and it doesn't has haste? Tap Maddening Imp and its gone at the end of turn.

Curtains' Call - a very great card. i usually play in a match with 4-5 players. that means i can destroy 2 creatures for just 2-3 mana.

Tragic Slip - Especially in EDH you got alot of turns where a creature dies. Tragic Slip allows you then to get rid of any single creature for B. Since its -/- you dont even have to care about indestructibility.

Tempting Licid - on the first look nothing special, but he allows you to do nasty combat tricks. opponent A attacks opponent B. pay G to add the licid as aura on a (preferable fat) creature A controlls. all creatures B controlles have to block that creature. This way you can get rid of alot of untapped utility creatures. And if the A's creature would also die (oops) just pay another G to end the effect after blockers are declared to get your licid back.

Tajuru Preserver - protect your board from sacrifice effects for just 2 mana. and hes way cheaper then his big sister Sigarda.

[WWolfe]

I had forgotten all about it until I saw it on Cmdr Vs yesterday....

Retribution of the Ancients - seriously does some work in +1/+1 counter decks. Since it gives -X/-X it gets around things that are indestructible (I'm looking at you Theros & Amonket gods) as well as effects that give creatures indestructible such as Avacyn, Angel of Hope or Sliver Hivelord. It does target so it does nothing to hexproof or shroud.

[WWolfe]

Thought I would resurrect this thread from the dead to try to find out what hidden gems people have discovered since the last posts over a year and a half ago.

One I recently discovered (just this morning, saw it on another site) would seem to be Ashes to Ashes. Same price as Murder with the additional cost of five life for exile of two creatures? Yes please. Definitely something I will be testing out.

I don't remember anyone mentioning Lapse of Certainty before either.   

ETA- moved to Commander discussion forum since this is almost entirely an Commander discussion.

[MonteTribal]

Reckless Spite is an instant speed counterpart to Ashes to Ashes., but does not exile. There is also Wicked Pact but it is just kind of worse version as well (but can kill artifacts!). In total, you can get 3 copies of this 2-for-1 into your deck

Llanowar Druid in mono green can be some nice ramp, not that you really need it in green lol. But it could be some sick pauper tech

Act of Treason, and most of it's variants, are great in Krenko, Mob Boss decks. I am personally fond of Wrangle at 2 mana and Act of Aggression because it is 3 colorless mana at instant speed, which is a nice surprise. It's not an instant speed effect usually

Furystoke Giant is also a fun piece of tech in Krenko

I know there is an entire archetype based around it (Momir Hack Ball) but I just learned of it recently:
Glamerdye, Trait Doctoring, etc on Momir Vig, Simic Visionary makes all of your green or blue creatures tutor for another creature. And then you can rinse and repeat to your hearts content.

[CleanBelwas]

One I recently discovered (just this morning, saw it on another site) would seem to be Ashes to Ashes.

I love Ashes to Ashes, especially in Commander. 5 life is such a small price to pay for that effect. And I love Drew Tucker's art on it too.

A few that come to mind from me that I don't think have been covered here are:

Spawning Pit. Everyone knows instant speed sac outlets are good, but 2 mana, goes in any deck and can make you more dudes to sac is just wonderful.

Undead Gladiator. I rate cycling probably much higher than I should, but this guy basically gives every card in your deck "Cycling 2BB", if a little slower. I play a LOT of black decks and decks that care about graveyards and this guy is a little engine all of his own.

Pain's Reward. This is my new favourite card. It's my favourite mini game to play in magic. The best part is, you always win.

Carnival of Souls. You have to be careful with it, but if your deck can handle it, it's amazing. Also has my favourite flavour text.

Faceless Butcher. It's fiend hunter in black. It has the good wording where you can sac it in response and the thing will stay exiled, but it's in black so you're in the best colour to take advantage of all the sacrifice benefits.

[WWolfe]

I looked out of curiosity and just found an Ashes to Ashes in one of my boxes. Can't believe I had forgotten about the card since I apparently own one. 

[WizardSpartan]

Steely Resolve - Fantastic in green based tribal decks. It's easy to run in 4-5 color tribal decks (Morophon, the Boundless or dragons, for example) also because it only requires 1 green pip. Also, it's only 2 mana and an enchantment so it comes down early and is hard to remove on its own.

Kenrith's Transformation - While it is a newer card, it definitely deserves a place in mono green decks. It replaces itself upon entering (fun little synergy with Yarok) and turns your opponent's big, threatening creature into a 3/3.

Dimir House Guard - Nice little tutor, sac outlet, evasive, graveyard synergies. It's a Swiss Army Knife of a card.

[robort]

Last Laugh has been my go to lately. A machine gun type boardwipe life loss effect. Granted it hits you to but it is really easy to start firing with it by putting almost anything into the graveyard