The "Seedlunky HD" mod automatically generates repeatable level layouts based on any entered text string. The added utility is especially useful for competitive purposes; players can distribute seeds among friends, subjecting them to particularly challenging levels. See if you can create a series of dark stages filled with tons of instant-kill spikes. Your friends will love you for it.
Spelunky already offers a similar feature in its Daily Challenge mode, which pits all players against pre-designed level layouts as they compete for leaderboard rank. Daily Challenge levels are only playable once, however, whereas Seedlunky HD-generated levels can be loaded and replayed at will.