Well, from introductory game let's jump for the deep water.
In my opinion, Stone Soup is the best rougelike available right now, it's free, has tolerable tileset (Believe me, some games have terrible sprites), and the amount of content is incredible.
More than 20 races, each one with it's unique abilities and skills, more than 30 classes to choose from, and more than 20 Gods (They do play a big part in the game).
Well, after being used to controls, prepare to die, a lot.
DOWNLOAD LINK: http://crawl.develz.org/wordpress/