Given my interest in obscure titles, some of the stuff I like might have not been well known in the first place, so it'd be hard to talk about "forgotten".
But I'm getting the impression that Will Rock is such an unduly forgotten game. Sure it's a Serious Sam clone in an Ancient Grome setting, but it does not play exactly like Sam, and in fact I don't think I now of any other game that would be exactly like that. I had a lot of fun with it back in the early-mid 2000s during my University years, played the campaign several times but never multiplayer though. I checked some MP maps in game host mode and they felt fun too, pity there were no bots to play around with.
The developers (based in St. Petersburg, Russia BTW) took a serious approach and had an expert or two on Ancient Greek and Roman architecture on board who consulted them on structure designs used in the levels. The game is nice-looking, fast-paced and fun to play, without getting too difficult or playing the gotcha game with teleporting monster traps and such. There are no particularly intricate layouts and most secret areas are easy to find, but almost every area feels unique and has its own design and theme.
Will Rock was actually the game that brought me back into playing 3D FPSs, because back then I used to think that anything after Duke3D was kinda meh. Mind you, at the point when I discovered the WR demo I had not played Serious Sam and only had a very vague idea of what it was (which is a shame because I love Ancient Egypt).
Annoyingly, WR is not available for purchase today AFAIK -- possibly because of hardware compatibility issues? I think Saber Interactive later made other games on the same engine but not sure.
There's a spin-off of sorts called God Mode (available on Steam) - a co-op MP third-person shooter which recycles many of the character designs (multiplayer skins) and monsters from Will Rock. But honestly from watching game footage I had the impression that it failed to recapture the original atmosphere, possibly the developers (unrelated to the original WR team as far as I can tell) didn't even intend to.