My favorite traditional roguelike is definitely Dungeon Crawl Stone Soup - you can play it from your browser!

Less traditional, but actually my favourite overall, would be Cataclysm: Dark Days Ahead - I say less traditional because it's more of a crafting game with base building elements than one where your goal is to find loot (although you can play it as a loot-finding game too - it's very flexible!). This one has to be downloaded.

And of course Dwarf Fortress isn't a roguelike at all, but I like playing with ASCII graphics so Honorable Mention.

They all have forums too.