The new Mac app Viewfinder by Connected Flow (developer Fraser Speirs is the man behind the FlickrExport plugins for Aperture and iPhoto) is designed for browsing/searching images on Flickr. This application provides many ways to work with the service's photos; you can select licenses to constrain your search to images you can use (such as Creative Commons or commercial use images), or search through the full text of tags and descriptions. Once you've found a photo, you can save a few steps in performing common tasks that would normally require you to download the image separately.
You can set the selected image to your desktop picture in one click. You can also insert the selected image into a Keynote presentation immediately. You can see the selected image(s) on Flickr, or you can download them. Also, you can get a "quick look" of the image by pressing the space bar on your keyboard, just like you would using the Finder.
If you're a frequent image searcher on Flickr, Viewfinder is a nice way to supercharge your search tasks. If you're not an image search pro, however, you're probably not going to need this application.
Viewfinder requires Mac OS X 10.6 Snow Leopard (which makes it Intel-only); Keynote '08 or later is required to insert an image into a Keynote presentation. It's available for a introductory price of US $23.60.