There are thousands of free WordPress themes out there, but I’ve found it better to pay for a theme.

  1. If you pay for it, you’re more likely to get support,
  2. If you pay for it, the theme is more likely to get updated as WordPress is updated.

Read this excellent article on WPMU

My favorite sources for themes are the following:

Customizing the Built In Themes

WordPress comes with two pre-installed theme that are easily modified.  Here are a few sites that use one of the two default themes.