I've had a lot of theme parties- mostly dinner parties, though. For my 16th birthday, I had an 80s themed party- everyone dressed up, it was great. I went as Smurfette.
For my 18th birthday, which just passed, I had a Chocolate themed party- it was sick. Everything was chocolate.
Hmm, some other fun ones have been Southern Comfort, The TV Dinner Dinner Party (dress up as your favorite tv star and eat homestyle food), China Town, Down in Mexico, Attack of the Carbs!, and Oooh la la- France!