This is a list of useful websites that I have come across (in no particular order just yet):

Related to Torchlight Mods

Torchlight
...is the official website of Torchlight.

Related to the Torque Game Engine

Garage Games
...the developers of the Torque Game Engine and it's various tools.

Related to WoW AddOn programming

WoWWiki
...is a useful site for just about anything related to World of Warcraft, including the WoW API.

WoW Interface
...is a site designed specifically for WoW interface and AddOn designers.

WoW Version Tracker
...is a site that provides in-depth comparisons of the different versions of the interface files released by Blizzard.

WoW Programming
...is a companion website for the World of Warcraft Programming book.

Lua
...is the website for the Lua language.

XML Nanny
...is the site for the XML Nanny program which helps with validating XML against schema.

Curse Gamming
... is a site that has a large repository of WoW AddOns.