Gamasutra recently interviewed the producer and director of Resident Evil 5.  The RE series is reknowned for its high quality, and the previous installment in the series, RE4, was many folks' choice for Game of the Year in 2005.  Resident Evil 5 will be the first RE for next gen (or is it current gen?) systems, which is notable in and of itself.  However, RE5 will feature co-op gameplay, which is a first for the series (not counting the two player co-op in the lightgun style offshoot RE: Umbrella Chronicles).  You might expect co-op to be a last minute afterthought, merely tacked on to the primary single player experience for such a title.  However, in this interview, the creators state that this is NOT the case:

For Resident Evil 5, a lot of the elements in it are a refinement of things that we had in Resident Evil 4; making them better, and choosing which elements to keep, which elements to discard.

The biggest change that we had to it, and the biggest change that we had from when we first settled on the design for it, was that we definitely wanted to have co-op in it, right from the start, and to have co-op be an integral part of the game, just right from the start.

So that's one of the first things that we decided, when we were making the game. That has led the development of the game. Right now we're about 70% of the way through the development process.

RE4 was certainly a great game, and the creators indicate that they will not try to reinvent the wheel for the new game.  Throw tightly integrated co-op in, and you have a game that we here at Co-Optimus are definitely going to keep an eye on.

Edit: The Resident Evil Outbreak series also featured co-op.  Thanks to TheReaperCooL for pointing this correction out.