It's possible - I have a bunch of ex's that I hang out with. However there are definitely boundaries on them that I have to follow. One occassionally brings up a girl he started dating after we broke up and I just start getting pissed off, so I stay away from that. The same guy also seems to have realized what a great catch I was since we broke up (this is years ago too...) and often attempts to resurrect that relationship. Since he's the one who killed my feelings for him when he managed to take me for granted, I stay waayyyyyyy away from that when he starts it up. Another is now dating someone who has a been a friend of his for years & years, so I totally respect that relationship and they are really good together. But I can't get too chummy with the ex because she's aware of our past relationship. Eh, it gets hairy when you look at the dynamics - particularly when there is a new gf / bf involved. But I am still good friends w/ these guys & know that I can rely on them in a pinch for everything that you would expect from a friend (but not a b/f).
I have yet another ex who I spend a lot of time chatting w/, but, again, I can't get into discussions about my private life or his because of things that were incomplete in our own relationship. We can joke about or discuss some very intimate things that are sort of hypothetical discussions, but if it involves a particular person or feeling I can't go there. Those are the bounds on that relationship. Some thing I know I can't rely on him to keep my feelings & best interest in mind, so I just don't let the discussions go there.
So yea, I think you can be friends with ex's, if they haven't hurt you in a way
that you can't forget or forgive. Others have boundaries, and others you can be complete best friends because you've bared your soul to this person and not been hurt for it.