Mac OS X has a screen saver that takes an RSS feed of images as a data source. Naturally I thought Flickr would provide good RSS feeds for tags, and users. Flickr offers these feeds, but unfortunately limits them to 20 items and does not offer any sorting or filtering options. To gain more control over RSS feeds of Flickr images, I made this Flickr RSS Generator. This generator uses the Flickr API to retrieve images determined by parameters you specify and returns an RSS feed, whose URL you can paste into the screensaver options screen in OS X.
tags tag mode all these tags any of these tags sort order date-posted-asc date-posted-desc date-taken-asc interestingness-desc interestingness-asc relevance maximum number of images to return
If you are interested in programmatically retrieving photos for your website or application, check out Flickr's API. It's free! You are most likely going to be interested in the flickr.photos.search method.