Dark Interval.. I got to play this recently. Solid, wish it was longer, but very interesting. It's not terribly fun but I think that's because it's authentic to beta HL2 as it was before the major revisions that brought to the concept retail HL2 uses. It shows precisely *why* the developers chose not to go in this direction. It's long, meandering, a bit tedious, dark and depressing. The atmosphere is ceaselessly oppressive, and that's just not as fun.
Recently got back into GoldSrc mapping.. and was upset to find out I apparently deleted all my old map projects. And with the Wadfather long dead, I can't even find most of the texture packs I used. The only thing I've been able to recover is this selection of textures that I used, but I don't know where most of them are from. R25 is identifiable but I can't find a download source now. wallcorrupt is from the Asylum wad I was able to get off the Sven Coop mapping site.