If I can say one thing it's that I'll probably never play this mod again just because it creeped me out so much.
You just walk across an island to various points of interest while some creepy narrative goes on in the background. Dear Esther - Not really a game but more of a short story in interactive form.
I think this mod utilizes zombies better than any source engine product I've seen, Left 4 Dead included. I for one up until now have mostly only played singleplayer mods, and I see a lot of those that end up under the radar while Minerva Metastasis get's a lot of the attention.
I guess we can just turn this into a thread where we suggest all our favorite HL2 mods.