Nox

Nox, by the now-gone Westwood Studios, was a gem of an underappreciated game. The single-player was good, with lots of neat spells and abilities, and a cool top-down line-of-sight system. But the multiplayer was quite unique. The multiplayer was a cool cross between Diablo and Unreal. It was still RPGish, with different character classes, abilities, and spells, but the gameplay modes were the kinds of things you'd find in Unreal Tournament—deathmatch, capture the flag, a sports mode, king of the hill, and so forth. It was action-packed, and it was greatly entertaining. It's a pity that it never caught on. Later on, they added a free cooperative multiplayer campaign too. It was extremely innovative at its time, received almost universal critical acclaim, and was made by one of the top development houses at the time. I don't really know why it didn't succeed. Maybe it just didn't get marketed correctly. Regardless, it's a sequel that I'd love to see and never will.

I'd link to a demo, but the demo is multiplayer-only, so you'd need to find some friends. It might have also been back in the day where multiplayer games were played over IPX, which is a pain to get working in some networks. You can buy the full game for thirteen bucks if that sounds interesting to you, though. :)