MustaKotka came up with the idea to make a thread with links to useful Commander resources that aren't found on Deckstats. Here's what we got so far (titles are hyperlinked), but if you can come up with any others let me know and I'll edit this post later. I won't be linking any other deck-building or forum sites. There is only Deckstats.
Commander Rules CommitteeThe official website for the people that make the rules of Commander. This website contains the official rules and the ban list.
EDHRECBy pooling deck lists from multiple sources, this useful website shows which cards are most frequently played with your chosen commander. Very useful if you have no idea where to start with building a commander. Furthermore, on Deckstats there is a tab called "EDH suggestions" found in the deck page. This tab gives recommendations based on EDHREC data.
ScryfallA search engine for cards that is more popular than
Gatherer. You can easily do custom searches to find whatever card or cards you need. You can also check the prices of cards, check the card rulings, download card art, and more.
PlayEDH DiscordIf you're looking to play commander through webcam play, this is the Discord server for you. Be sure to properly asses the power of your deck using the PlayEDH guidelines, to ensure that the game you play is fun for everyone.
Commander SpellbookA comprehensive database of combos in Commander. New combos are constantly being added in. If you're looking to add a combo to your deck, the search engine will help you out.
cEDH deck list databaseA list of the decks from the competitive side of things. These decks are top-notch, the best of the best. There are also fringe competitive decks that don't quite make it to the top spot.