Do you want to compete against your friends at Spelunky, the randomly generated platforming roguelike? That's what the Daily Challenge is for; each day, a single set of levels is generated which is the same for everyone and which can be played only once.
But if once isn't enough to satisfy your competitive urges, there's now Seedlunky HD. The user-created tool lets you set a custom seed from which to generate levels, which you can then share with your friends while you continue to compare your adventures.
Used (somewhat) maliciously, this tool could help people create unfair leaderboard scores and speedrun times. Used for its actual purpose, it could be an interesting tool for new players to practice the game's perilous worlds, and a fun way for the community to share unusual seeds. Spelunky's level generator is focused on creating distinct but achievable challenges, so it's unlikely that we'll see anything visually outlandish, but I'm curious whether player will focus on the hardest seeds - this one has two dark levels, etc. - or the most profitable.
The tool also lets you enable 64-bit seeding, which provides a larger number of random levels than Spelunky HD. That's because it "will bypass Spelunky's 32-bit Mersenne seeder and use its own Mersenne array init function to seed Spelunky's Mersenne state with a 64-bit number instead." I don't really understand some of those words, but it sure is cool sounding.
For the curious, Darius Kazemi wrote an interesting breakdown of the original Spelunky's level generator.
Check the Seedlunky HD forum thread on the official Spelunky messageboards for a download link and more instructions.