Maybe this is a lot of nostalgia but I have a few:
Poy Poy 2 on PS1. I have had a hard time ever finding anyone who even played it but we had tons of fun with it at home playing it 4 player free for all.
Rise to honor on PS2 I think that was the name. I don't remember the story but to fight you basically used the right stick in different ways which we at the time thought was really cool. A bit inaccurate but unique and we had lots of fun with it. It got slammed in reviews I have since gone back to read.
Dofus on PC. It's a french turn based MMORPG that I played as a kid. Found an ad on a website called miniclip but I have yet to see any ads since then. I played it what must've been over 10 years ago the first time and I played it for many years. I have recently picked it back up and while almost everything has changed I still enjoy it quite a bit, the (english) community is pretty friendly too although it is a bit small.