Phantasy Star Online
I would say this game would definitely be one of Borderland's influences. For those not in the know, PSO was the first console mmo and it played a lot like a 3D futuristic version of Diablo and it was revered for being one of the best games of it's time. Sadly the Dreamcast died but PSO lived on as it was ported to many consoles like the PC(in the form of Phantasy Star Online Blue Burst), Xbox, PS2, and even the Gamecube! It is massivly addictive without being life sucking and its got four player online co-op hence the word "online" in the title. Sadly all the servers shut down but I found a private server about 8 months ago called "schthack" so people who own PSO for the dreamcast, gamecube, or a free download of Blue Burst could play it again(and for free too! no more monthly fees!) and its just as addictive as it was in 2001. If you want to know how to get it, here is a helpful video: http://www.youtube.com/watch?v=sY0EkldfwOk&feature=related and if you haven't played it and like games like Borderlands(which is kinda why you are here eh? XD) then I HIGHLY reccomend it. Anyone here play/played it?
PSO was one of my favorite games! I sunk so many hours into it! I occasionally play on schtack but not often. Borderlands reminds me of PSO a lot. I think that is one reason I'm so interested in Borderlands.
colorblindmonk: Schthack then might be better for you, it disconnects a hell of a lot less often and though the controls still feel weird they become second nature when you get used to them(plus you can use the numeric keys to hotkey abilities, handy if you ask me).
I think your thinking about Phantasy Star Universe, I only play the demo for that one cuz as it stands, its not worth paying for :/
I play the demo on the Xbox 360, its fun but everytime you try selling or buying stuff the laggers steal it. Still its fun, I was thinking about getting the full game for xbox 360 but not online. Is it still fun without online?

I tried to get into it, but clunky controls and a good deal of lag and network errors got in the way. Probably because I was playing with British people.