There's KISS: Psycho Circus, to some extent (lots of enemies on-screen), but it's boring overall, although the level design is generally decent, I haven't played nor even installed it in years, I bought it back in 2001 I believe, or perhaps before that I can't remember anymore, but it was released in 2000 if I recall correctly. There's unfortunately few games "like" Serious Sam, other than Painkiller as mentioned by shortylickens, and while we're at it there's Painkiller: Resurrection coming out soon, it's been announced on Steam last week, it's basically like the original but with better graphics, it's always a good thing. And, well, the classic, DOOM series, especially DOOM II, it has more enemies in most levels than in the original but it's still not quite what you're looking for.