I personally haven't named my car...but my wife has.
Sometimes it is "The Red-Headed Mistress"...and when I, according to her, spend too much on it it is "The Red-Headed Slut"!
First car was Benson, current is Benson Jr. I threw the name out there while in the car with some friends and it just stuck. I figured why change it for the current car.
My girlfriend's named her GTI Bianca - she found a list of popular names on http://www.namesforcars.com/ - apparently there's a site dedicated to car names and also October 2 is Name Your Car Day ?!? Anyway I think is nice to give a name to your car.