Way back when, OSWD.org was the place for free templates. Most were public domain or had GPL-type licenses, and it was a great way to get a site up and running with an attractive layout.
OSWD hasn’t been actively maintained for a long time, but OpenDesigns.org and GetFreeWebDesigns.com have replaced it. Quality varies; some are amazing, some are mediocre. But for the price, you can’t really complain. It’s worth noting that many are not free (as in speech); they often require attribution or similar. This may or may not matter. (For something like my site, I gladly left the linkback in the footer. If I were designing a commercial site, I wouldn’t be so comfortable with that.)
I bought a template from OneDollarTemplates.com a while back. I was pleased. (Especially given the price!)
Today, I just discovered Theme Forest, where a lot of extraordinarily talented designers sell excellent templates for low prices. Columns is an amazing blog template, for example; Nova and Liberation are quite something, too. And Blue Business. Dezineplus is superb. Really, most everything there is pretty fantastic.