So, what's a Roguelike? Rogue was an ASCII dungeon-crawler written for Unix systems in 1980. Since then it's inspired literally thousands of similar games - genericly called Roguelikes. Every year there's a competition to see who can devise the most interesting new variant in 7 days - The Seven Day Roguelike Challenge.
One Roguelike in particular has been under continuous development since 1987. If you don't mind wrecking your grades/life/marriage/career, download The Greatest Game You Will Ever Play from the project's website [nethack.org].
Here's a dime-bag to give you a taste. Use the arrow keys (or "jkhlyubn") to move your intrepid explorer - "@" - around a dungeon. Type ">" to go down any staircase (">") you encounter.
Just reload the page to play again when you're killed.
(If you want to go deeper, play the version on the "Roguelike" page above.)