Playable beta versions of Unreal. Not all of them have levels, some have only test maps or don't have any maps. On Windows Vista and newer run as administrator (you need to do this only once). Some versions have two executables, Unreal and UnServer. If Unreal does not work, try running UnServer.
Full game, close to release. Not too many differences, mainly in DmSolaris and QueenEnd. Possibly a fake.*
There's also a build known as Unreal.098.1997.11.03, but it's confirmed to be fake.
*Suspected by some to be fake, but there's no consensus. The evidence isn't as clear as in the "098" case. The only thing I can say with certainty is that if it's a fake, it wasn't my work.