Most of the information here is for Warzone 3.1 and above.
Terrain Tilesets
General
http://forums.wz2100.net/viewtopic.php?f=33&t=2383 – lots of textures and experiments dating from 2008 to present day
http://developer.wz2100.net/wiki/ModdingTilesets – guide to modding tilesets
Warzone 3.x
Unsorted:
http://forums.wz2100.net/viewtopic.php?f=49&t=10451#p115420 – Alaska & Desert tileset for Warzone 3.1 (work in progress)
Screenies for cam4 terrains: http://forums.wz2100.net/viewtopic.php?f=3&p=115717#p115680
NoQ's "palace" (unsure of name) tileset
Warzone 2.3.x
The Arizona, Rockies and Urban tilesets below are based on the native tilesets from Warzone 3.1. If you want to convert Warzone 3.1 tilesets to Warzone 2.3.9 format, try NoQ's conversion script.
Unsorted:
http://forums.wz2100.net/viewtopic.php?f=30&t=9667 – lush green and desert tilesets for Warzone 2.3.x
Warzone 1.x