The first roguelike I discovered was Nethack, in which you are an adventurer on a mission to recover the Amulet of Yendor for your deity.
I also play Slash'EM, a Nethack variant with lots of extra classes and races, new special levels and other new features. Slash'EM can also be downloaded from Ali's Slash'EM page; if you are in the UK you'll find this page much faster than the official Sourceforge site. I have written a collection of Slash'EM spoilers; links to them all can be found on my page of differences between Slash'EM and Nethack.
Another good roguelike is ADOM (Ancient Domains Of Mystery), in which your aim is to save the world of Ancardia from the invading forces of Chaos. ADOM has a much more complex plot than Nethack or Slash'EM, featuring many useful unique NPCs and multiple ways to win the game, but its user interface is not as friendly as Nethack's. For example, if you read a scroll and it isn't automatically identified, there is no way to name that type of scroll, unlike in Nethack. Here are more comparisons of Nethack and ADOM.
The best places to go for ADOM spoilers are Andy Williams' ADOM Guidebook and the ADOM Forums (web-based discussion forums where I post as Ekaterin). You can read other people's gameplay tips and add your own at the ADOM Wiki. Here are a few spoilers I have written. These were originally written for version 1.0.0, but should be applicable to the current version.