Hexen 2, Blood 2 : I sadly never came across these games.
I was always a fan of quake 1/2 based gaming, which HL1 mods as a very close family.
I remember when online gaming was synonymous with HL1 mods, then I had some fun with warcraft 3 custom maps (mostly in lan). nowadays there are two categories of multiplayer games, MMOs and "your PC isn't powerful enough to play it"
I'm exaggerating a bit, left4dead did a good job at running on crap, but back in the days if you had a PC powerful enough to play divx then you could play the popular, skill based games.