There are, that I've
noticed, three different
archetypes of ex-girlfriends:
1: The Dumper
This is the girl who walks up to you one day and tells you it's over. Also commonly known as "The one who broke your heart." The creation of this being is brought about by such things as a mid-life crisis, a mere re-evaluation of you, or the poorly hidden letters from your lover which she accidentally happened upon. You often hold no grudges against this type of ex, and can often manage a good conversation if you bump into her on the streets, providing you have done nothing to make her queasy at the site of you. Depending on the harshness of the dumping process you will probably still love her for a while, this will fade after a time, generally a long one.
2: The Dumpee
"I would be happier if I didn't have to inhabit this planet with you," of course, with a little more tact. This is often the case where you have recently discovered that your girlfriend is either a slut, an overall cruel person, or just plain ugly, (it could also mean a change in orientation on your part). When you are in this scenario you generally don't ever want to see the person again, as it is a reminder of the stupid mistakes that you are just too perfect to make. I often prefer being in this situation, as there is less chance of having a broken heart, even though you might cause one.
3: The Mutual Friend
This is probably both the rarest and most sought after type of ex-girlfriend. Often caused by things as simple as a change of pheromones. Relationships that end as mutual friendships usually never reach the love stage, or they have reached it to such a degree that the love doesn't leave, it just changes into something different. This could very well end up being your best friend for most of your life, and as a further benefit you will probably be able to discuss much more personal stuff with her then you would have been able to if the relationship had never gotten to the 'couple' stage.
Note - Not only is this a bit exaggerated for effect, but it is also taking it solely through the eyes of the male in the situation.