There has been a MP minimod of HL2!
I have it, but havn't got it working yet...
no time...
here's the link though:
http://www.hl2files.com/file/HalfLif...Mini_Mod;33099
It's a mini mod with 2 maps. A physgun map and a deathmatch style map. neither of which are very good. I'm thinking of converting my current project, he_city, for HL2MP.
Also, there are a ton of planned mods listed, many of which have their sites hosted by hl2files.com