Spelunky is brilliant . Part of the reason it's brilliant is the random generation, ensuring that each of your many retries are a chance to explore something new. Even the Daily Challenges - which randomly generate a new level each day, and share that level between every player - are brilliant. Here, then, is a community application that lets you completely bypass those randomised levels by setting your own custom creation seed. Will it still be brilliant? It's Spelunky, so probably.
The utility - found on the Mossmouth forum - lets you force Spelunky's level generator to use a predefined seed, which you can then share to other players in the spirit of friendly competition. Alternatively, you can go in the opposite direction: telling the utility to generate a 64-bit hash, thereby ensuring a bigger pool of random levels to draw from.
All of which sounds great, but opens up the possibility that players can gain an advantage on the leaderboards. By finding a seed that has beneficial items early on, or just by being able to practice on a non-randomised level set, it gives less-scrupulous high-score chasers a leg up on their honest competition.
Used in good faith, though, it's a fun little tool. And, short of an official, leaderboard skipping alternative, it's the only way you'll be able to create a special campaign challenge between you and your friends.