I recently found a treasure trove of website themes over at Open Source Web Design. I found one I liked and decided to try my hand at making it work with Drupal. The process isn't tooo bad, but did take a bit of work to get to what you see now. There is still work to be done, but mostly on the Drupal administration pages. And this is simply adding padding, removing some borders, adjusting spacing, etc. Typical CSS things.
Overall I'm rather impressed how easy it is to take a CSS based theme and make it work with Drupal. Some PHP knowledge helps, but isn't really needed if you can figure out what parts of sample theme to cut and paste into the new theme.
Of course, Drupal being Drupal, that particular "theme engine" is only one of three supported out of the box. You might find it easier to use one of the others. Take a look at the Drupal Theme Developers Guide if you want to do more.