Vowl by Steven Frank is a straightforward Mac app, available for free in the Mac App Store, that generates an automated Flickr slideshow based on tags assigned in the app’s preferences. The app is really simple with its translucent black window style, and also allows you to click on any picture to get to the Flickr photo page with your default Mac browser.
In the preferences, you can set the delay between photos, add and remove tags (by default, the app comes with “cat”, “dog”, and “chicken” tags) and make the app float above other application windows. When resized to take the most part of your screen, Vowl becomes a pretty effective way to get random Flickr photos on your desktop, and I have to admit I loved the kitten ones.
The app is a free download here, or on Steven Frank’s website.