Daisuke Shimamoto @ Yonezawa Lab


Homepage by Greg Hoglund, author of Exploiting Software. Many works done on rootkits. Mainly for Windows.
Many articles on hacking, cracking and so on. The famous article on buffer-overflow, "Smashing the Stack for Fun and Profit", by Aleph One can also be obtained on this site.
Undocumented Windows NT
You can read this book on the inside of Windows for free. As the title suggests, many of the contents have never been documented by Microsoft.
VX Heavens
You can download many viruses here.
Fairly new site about rootkits, exploits and reverse engineering. Has many useful stuff if your working deep inside the kernel of Windows.
Has a lot of exploit codes for various kind of software vulnerabilities. (Executables, Web Apps, etc.)