Part of the point of Spelunky is that players never see the same level twice, always learning techniques to defeat what the level generator throws at them rather than simply layouts. But if you do want to replay a particular world over and over, for whatever reason, a canny little hack made by one fan will let you force the generator seed and recreate the same set of levels over and over.
Some notes on how Spelunky generates levels... for every individual level, like 1-1, 1-2, and so on, Spelunky seeds a Mersenne Twister PRNG with a 32-bit number grabbed from the system timer (QueryPerformanceCounter). Enabling a custom seed causes Speedlunky to force the same 32-bit number -- the hash of the entered string -- to be used for each level in the game. (Or 64-bit hash, see below.) But won't this mean two levels in the same area could repeat? No, because Seedlunky also adds the index of the level to the seed.
The generator cannot, however, interfere with the Daily Challenge. Players using the tool could share the same run as all their chums, run, practice the same set of levels over and over to create an amazing run full of wonder and near-misses, or pretend they did it for real and boast about their high score like a pitiful wretch.