GNU (Ok, so technically "GNU's Not Unix" - it's done by unix literate people)

• www.gnu.ai.mit.edu
• GNU Hurd information
• The Unofficial GNU Hurd page
  • Programmer info
  • Policy info
• DOC for GNU utilities

*BSD

• Lites Home Page

Pure mach

• The Mach 4 Project
• Mach, and experimental OS'

Other microkernels

• L3 and L4