RogueBasin - Roguelike Development Wiki
DungeonDweller - Amongst other things, contains a collection of useful algorithms for roguelikes. A great resource for developers.
Simugraph development library - Contains some algorithms for development, not as much as DungeonDweller but there are a few good things there.
Roguelike graveyard - A home for abandoned roguelike projects.
Roguelike Restoration project - A project dedicated to making Windows/Linux ports for old Rogue variants.
ADoM - A great Roguelike game. Inspired me to make a roguelike myself!
NetHack - Another very popular roguelike game. Perhaps the most popular.
Linley's Dungeon Crawl - A somewhat-popular roguelike game with much simpler gameplay than ADoM/NetHack, yet still fun to play.
Angband - A roguelike game with many variants.
T.O.M.E - My favourite variant of Angband, also probably the most original one.
Moria - A classic Roguelike from the 1980s, Angband is based on its source code.