Probably you know the glslideshow-Screensaver from Linux (also on OS X):

• A1:1 conversion of the screensaver is Nostalgic by Gregg. Set image delay to 24sec and a transition of 5sec to run it smoothly.
• The MotionPicture-Screensaver is similar. You better also tweak the setting here or it might run to fast.
• Microsoft has also a similar screensaver, but it cannot read subdirectories of your image folder. You may consider to download the Images of Ireland (Validation required) which includes the screensaver instead of paying $20
• Last but not least you should have a look at the Google Photos Screensaver from the english Google Pack. You won’t see it in the german Google-Pack, probably because it is fed by Photofeeds like the one on flickr. Anyway i encountered two or three problems with the screensaver, not wanting to switch back from the OpenGL or DirectX-Mode on exit and so let me see my desktop again. The only chance was to reboot. Also i dislike not beeing able to switch of the info-bars and the Google-logo and i also don’t like also seeing mini-sized photos.

Here are some feeds i found interesting:

• Earthshots.com
• flickr: Club pictures
• flickr: Club pictures II
• flickr: Club pictures III (Ministry of Sound)
• flickr: Erotic Photos I – Playboy Bunnies I
• flickr: Erotic Photos II – Playboy bunnies II
  
• flickr: Erotic Photos III – Sexy Models
• flickr: Erotic Photos VI – b/w
• flickr: Polar and Arctic Pictures
• flickr: Photos from Lone Cypress
• LiveScience.com – Image of the Day
• LiveScience.com – Amazing Pictures
• PicasaWeb from Google.com
• Space.com – Image of the Day
• Space.com – Amazing Images

Anyway, it’s easy to find more pictures:

http://api.flickr.com/services/feeds/photos_public.gne?tags=<your tags here, separeted by comma>&tagmode=all&format=rss2