I've actually been playing a lot of Borderlands co-op again recently. When I first played it I guess it didn't have any of the Steam achievements, as I've been getting loads of unlocks and I completed the game on my first playthrough. You also couldn't play it online co-op when I first tried, so that's a big improvement too.
Really looking forward to Borderlands 2. RAGE is looking good as well and it's so rare that games have co-op it's pretty much a must buy.