Lambda Land right now:
Pre-Release 1
Future:
Pre-Release 2,3,4, & 5
Prototype
Alpha 1 & 2
Beta 1 & 2
Release
What happens after that?
Xen's Horror Release
Half-Life: Outpost - Alpha/Beta
Portal: Issued
Xen's Horror: Source
Everything after that is 100% unkown.