Here is the full list of all 20 Spelunky achievements worth 400 gamerscore. It takes around 80-100 hours to unlock all of the achievements on Xbox 360.
Completed the game without collecting any treasure. No shortcuts.
Completed 100% of the journal.
Completed the game with at least two players alive. No shortcuts.
Completed the game in under 8 minutes. No shortcuts.
Obtained 500000 gold.
Completed the game the hard way.
Rescued all 8 hidden characters.
Completed the game without using shortcuts.
Played Adventure Mode 1000 times.
Reached the City of Gold.
Killed 12 or more shopkeepers in one game.
Rescued 10 or more damsels in one game.
Completed the game.
Obtained the Ankh.
Won the kapala from Kali.
Reached the Temple.
Completed 50% of the journal.
Reached the Ice Caves.
Reached the Jungle.
Beat the Tutorial.