Nethack (wiki) is a classic roguelike that is an absolute must play for those new to the genre. Roguelikes are typically dungeon-crawling turn based RPGs with a ASCII user interface and Nethack is no different
While many might consider the graphics to be archaic in comparison to modern counterparts, great depth lies underneath this seemingly primitive interface. There are numerous spells, potions, armor, and weapons to collect and equip along with 13 classes and 5 races (many with unique traits) to choose from.
You can download the standlone here, you can play on a public server by pointing an ssh client (If you're on Windows, give Kitty a try!) to [email protected]. You can even use your browser to to play alt.org by going here.
Need help or fancy yourself a chat? Point your IRC client to #nethack on Freenode.
Check out the The Greatest Game You Will Ever Play if you're in for some more info (and a sensible chuckle)