I have actively participated in numerous Microsoft Beta's in the past.. noticibly:
1. Windows 2003 Server Edition
2. Microsoft Office 2003
3. Windows Messenger 5.0
4. Windows XP SP2 (When it was private)
Currently am participating in afew betas, but we wont go into any details
. Its actually a fun job.. You have access to pre-release software, and usually have some fun whilst testing and reporting your bugs on the newsgroups that Microsoft provides.