Google 'Free Font Download'
When in doubt, Google it out. There isn't one set font that everyone uses in their avatars/signatures. I personally don't like using a font twice unless I have to (i.e. self-drawn pixel font in my signature) just for the sake of variety. Typically, you'll want something that fits the theme of the image, such as a digital/LCD font on a circut board/computer type image, or a flowing script on a fantasy type image. The best idea is just to have a nice sized collection (I have 271 currently) so that you can find the one that best fits the image you've created.