Super Meat Boy devs Team Meat are planning a hub for user-created content: Super Meat World. The hub's launch is set for "very soon, days away even," according to tweets from Team Meat. Aside from allowing Steam players to share and download individual levels, Super Meat World will support sharing of full chapters -- at least those that Team Meat chooses to feature.
And once Super Meat World is complete, the team says it'll get to work on the Mac iteration of Super Meat Boy, with a Linux version following that (seriously). As previously noted, Steam users who already purchased SMB won't have to repurchase to play on OS X, as the game features SteamPlay functionality.