+1 Excellent recommendation; I picked up a few Serious Sam games back in the day and they are totally new 'Dooms'.
IIRC the "plot" involves time travel, aliens, and how fast you can shoot them. Loads of fun, pretty difficult, decent 3d graphics, none of that other bullshit.
Painkiller is based along similar lines but retains Doom's darker demonic palette whereas Serious Sam is very light-hearted and colorful. A number of people have mentioned that it is essentially what Doom 3 would have been if ID hadn't changed the series' genre to horror rather than staying an action shooter.
https://www.youtube.com/watch?v=EmIzCztjSRQ