6.2.0205 =>
Official Public Version
Anything higher is Beta, meaning it is in test-fase.
7.0.0429 =>
Current Public Beta Version, meaning it can be downloaded and used by anybody. (
http://messenger.msn.com/beta)
Anything higher is
Private Beta, meaning it should only be used and be available to beta testers. Using it as non-beta tester, or leaking it is illegal.