Here's what I came up with looking for the best card in each of my EDh decks that I haven't seen played by anyone else:
Power Conduit: I've used him with Arixmethis, Slumbering Isle to get him down two counters the turn I play him, and then another at the beginning of the following turn. I've used it with
Decree of Silence to counter an extra spell.
Devoted Druid can tap for 3 every turn. Any time you find yourself needing counters removed, it's surprisingly good.
Shadow Rift: Making a creature unblockable for 1 mana is good, having it replace itself is even better, but having the flexibility to also make a creature unable to block instead is what really makes this card undervalued.
Instill Energy: I don't know why this is so underplayed. Haste in green is nice, but the ability to untap each turn is awesome. Your mana dorks tap for double, basically paying for the spell the turn you cast it. If you've got a big boy that you want to attack with but need to hold up as a blocker, here's psuedo-vigilance. It's versatile and cheap.
Spectral Searchlight: This is okay-at-best ramp, but it's a great tool for politicking if you're into that. How many times has another player had an answer to a threat, but they didn't have the mana to play it. If you have something scary staring you down with no way to stop it, it's super easy to say, I'll give a mana of any color to anyone that can remove this threat before it hits me. The rest of the table will have the deal with the threat soon enough, so they're usually happy to step in if you help with the cost.
Soul Foundry: This can really fill a lot of roles. From making an essentially free
Llanowar Elves every turn (since the one you made last turn pays for this turns activation), to finally getting triggers from
Charmed Stray's ETB, there are endless ways to make this card interesting.
Dimir House Guard: Free sac outlet and a tutor. Why is this not played more?
Awe Strike: One mana to turn any instance of damage into life gain. This can really wreck players. Aetherflux Reservior? I'll let you pay 50 life to have me gain 50, that seems fair. Going all in for a one shot kill with commander damage? I'll be happy to gain 21 life.
Heartless Hidetsugu? Yeah, I'll gain half my life total instead, hope that's okay.
Sisters of the Flame: A 2/2 mana dork for 3 isn't that exciting, unless you're in red. Red desperately needs ramp, and it loves getting a lot of little creatures on the board anyways. (She's played in less than 100 decks on EDHrec?)
Skybind: This is a fabulous way to recur ETB's and can even be used before a board wipe to protect a key creature or, with an enchantment with
flash, can save a permanent any time. White loves enchantments and ETBs, so this would be a great fit in so many decks.
Rishadan Footpad: Having everyone sac a creature is good, putting it on a recurrable body is great. Yes, they can pay their way out of it and they choose the creature, but with a little patience or planning, that shouldn't matter.
Inviolability: Most card like this prevent all damage that would be dealt by and dealt to enchanted creature, or just deals with combat damage, but this just prevents all damage dealt to the creature. This has plenty of ways to abuse. Maybe you have an attack trigger that doesn't matter if you get through or not, maybe you have a creature with infect, but it will die the first time it tries to swing in, maybe you have a
Wall of Glare or the like and just want to block everything, or maybe you're about to play
Blasphemous Act. This is gold in every case. Even if you just put it on your biggest creature and swing every turn, that's still quite good.
Thundering Djinn: There are lots of ways (especially in blue) to draw 10, 15, 20 cards in a turn. This turns that card draw into damage when it attacks. Even if it doesn't survive, it still gets that damage in, so it can always turn a bunch of draws into damage, but it does have evasion, so it will often get through as well. There are 17,000 decks on EDHrec playing
Psychosis Crawler, and yet less than 300 running this guy. Obviously, this isn't as good, it only counts card draw on your turn before combat and it only hits one target instead of each opponent, but it can also direct that damage at a creature or planewalker, so it does have a little versatility that the crawler doesn't.
Well, there you go, a hidden gem from each of my decks. Not many of these should be auto-includes for any color or archetype, but they're all fairly cheap and off the beaten path options that can be really great in the right build.
Let me know if you end up using any of these guys.