I just played Estranged act 1 on steam. You do not need HL2 installed to play it. It is also free. and has source code as well, I think.
Basically you are the fisherman on a boat and must unfortunately get off the boat to inveshtigate an island. It is supposedly a horror mod, but I've played black snow and no mod can get scarier than that hands down.
It's not very scarey, honestly, and certainly isn't the best mod ever either. But it does offer slightly different play than the average HL2 mod, seeing as your enemies are all human and dont look like combine soldiers or headcrab zombies. There isn't that much combat either, and get this, you can only carry a single weapon with you at all times. Do I recommend this? Sure, if you want a quick revisit to the source engine. Wouldn't put it on any sort of priority list or anything.
(This was more of a review and less of a summary, which is what I typically did. Oh well.)