Since I unfortunately don't have time to personally review all of these great choices at the quality I'd like, (or greater than has already been done), I'll link to the best ones I've found below.
The Undercard:Lots of fun and interesting choices here, and these projects need your help!
- Haiku Alpha 1 vs.
This is an Open Source version of of the amazing "hyper-threaded" BeOS.
- ReactOS 0.3.11
Teaming up with WINE to deliver a complete open source Windows compatible OS.
- Ubuntu 9.10 (Karmic Koala) vs.
The primary Linux contender for the desktop.
- OpenSUSE 11.2 vs.
A recently updated "enterprisey" robust GNU/Linux operating system.
- the BSDs
Everywhere and yet nowhere.
Saturday Night's Main Event:
- Mac OSX Snow Leopard vs.
No OS is perfect, but this one has a some big advantages. It's hard to beat Apple/NeXT GUI magic with UNIX underpinnings.
- Windows _7_
Hasta La Vista, Baby! The best release of Windows in many years.