I have R2-D2 as my default ring tone. In the past, I've used "Fruity Oaty Bars" from Serenity. I also have a cat meowing, several songs that I've edited bits out of to use as ring tones, goats bleating (it sounds like an evil laugh), one that sounds like hamsters playing scrabble (I don't actually use it, just have it for giggles), a Mario games death sound, George Takei saying "Oh Myyyyyy", and the Transformers transforming noise. I have several people who have their own ringtones (for Mom and Dad, it's the Muppets doing Mahna Mahna), and I switch out the default ring every so often.