Cheats and Hints

Absolute Beginner's Guide for NetHack 3.4.1
Basic primer for new players. Does not contain spoilers.
http://www.melankolia.net/nethack/nethack.guide.html

List of NetHack Spoilers
Collection of spoilers relevant to 3.4.3 release.
http://www.statslab.cam.ac.uk/~eva/nethack/spoilerlist.html

NetHack 3.4 Spoilers
Spoilers, NetHack Gazetteer, NetHack Code, Usenet FAQ, and miscellaneous help.
http://www.spod-central.org/~psmith/nh/

Ye Olde NetHack Bestiary
Statistics on creatures encountered during gameplay.
http://www.cim.mcgill.ca/~simra/nhtohtml/html/

Yet Another NetHack Site
Spoilers for versions 3.3.1, 3.4.0, and 3.41, including those posted to Usenet.
http://www.steelypips.org/nethack/