Looking at the stats on this site, WordPress Themes seems to be a real hot topic right now. Lots of people seem to be searching for that term.
I guess with the rollout of WordPress 1.5 on the 15th and the native ability to use Themes built in to it, it is becoming something of growing interest to all WordPress users.
There are a couple of excellent theme sites out there – check out:
The official resource for v1.5 themes (thanks MacManX for the reminder),
The UNOFFICIAL WP Themes List for 1.5
WP 1.5 Themes
If you know of any more sites with great WordPress themes, be sure to let me know and I will list them here.