Borderlands is a very engaging RPG/FPS add that with multiplayer and its an very exciting game.
There are 3 DLC for the game the most noteworthy being The Secret Armory of General Knoxx which is practically a sequel in its own right. This raises the level cap from 50 to 61 so if you don't have it yet you should get it. This game is very fun because each time you play you'll always get something new like you might not have that epic smg and instead use a shotgun or sniper rifle. Then there are 4 different classes that use different styles so you can invest a lot of time depending on how far in you want to go.
Like theres the Siren who can use her move the phasewalk and can go in a different dimension making her super fast and then her skills might allow you to do damage to people while in it. Then theres Roland who can deploy a machine-gun turrent that can either be used for dmg or healing people. Mordecai can release a bird that can swoop at people while snipes or shoots them. And Brick can just kill anything in sight with his fists and tank.
So in Borderlands theres a to of things to do and the online is still pretty active so if you haven't gotten this game yet get it now!