securityfocus.com
Possibly the best online resource for computer security.

labmice.net
"The definitive online resource for IT Professionals who deploy, manage, and support Microsoft Windows 2000 products and services." They're probably right!

The XML Cover Pages
A comprehensive online reference work for the Extensible Markup Language (XML) and its parent, the Standard Generalized Markup Language (SGML).

gotdotnet.com
Great place to start for anything about Dot.Net.

allapi.net
If you're from a VB background, you might like this site, which gives the ".NET way" of doing the same things you could do in VB.