I live in the middle of nowhere, so i've hit birds, rabbits, hare, rodents of all kinds. What amazes me is that how stupid pheasants and pigeons are compared to crows and the like. A crow will pretty much always get out of the road well before the car is near, even if it's feasting on the remains of something really tasty (like the stupid pheasant that didn't move out of the way). However, pigeons have to be the dumbest birds on the planet and will invariably just wander around in the middle of the road with no concept of preserving their own lives.
As much as i hate hitting stuff, my most memorable was similar to a previous poster. I was being tailgated by a guy in a white BMW coming from a single track road onto a dual track. I moved into the outside lane to overtake at about 70mph (ish) in my Honda S2000 with the BMW driver just feet from my rear. Its amazing how much a full sized male pheasant gets mashed up when it goes underneath a car with a low ride height. The BMW driver learned a lesson that day (actually, i doubt he did). His car turned a nice shade of red after the incident.