SMOD for Half Life 2.
More specifically, Outbreak and the impossible to find version of Remastered I use(a version from before it tried to imitate Tactical.
Outbreak is a form of zombie invasion mod. More specifically, the creator of the mod(in the old version I use) ripped sound files from Doom3, and new 'zombies' include the Hell Knight from Doom3 and one of those mutant enemies from Shadow of Chernobyl(the thing with tentacles coming out of it's mouth). I'm not completely sure what's original and what is stolen but I usually played the combine sections because of the limited availability of weapons(7 weapons total, rocket launcher, 3 assault rifles and 2 or 3 handguns) which was great in combination with the stronger combine forces(both more combine and elite versions).
Remastered, or Remastered Redux(version 5 from late 2007 IIRC). It simply added more enemy types and a lot more weapons to SMOD. It also added new player modes for use in third person mode. The obvious models like Alyx and Gordon Freeman were available, but also a Stormtrooper, Lara Croft, the Frohman model, Master Chief and what I think is HUNK(Hunk?) from Resident Evil mercenaries. I know I'm forgetting some but I haven't played it in months(not surprising since my video card died this summer and this new thing hates games)