I've recently been playing around in Flickr and have seen quite a bit of additional traffic to my site from Flickr referrals.
Plus it's nice to get a little feedback on your designs and creations and check out the work of others!
What design/illustration groups are you a member of?
I currently add the odd photo to;
Bittbox User Showcase
PSDTuts User Showcase
GoMedia User Showcase
Design and Illustration