Viewfinder - a Mac app for finding the right image on Flickr
Viewfinder also has Keynote integration (for Keynote '08 and above), for folks who like to use Flickr photos in their presentation slides. You can drop Flickr photos right into Keynote with no mess, instead of downloading and opening the files and trashing them when you're done.
Viewfinder carries a £15 introductory price -- that's roughly 25 US dollars -- but it could easily be worth the cost for people whose workflows depend heavily on Flickr. If you want to test it out, the free demo has no time limit, but it only allows 2 downloads per launch, and it delays each download by 30 seconds.