Android: Flickr's own Android app leaves a lot to be desired. Glimmr, a new app from XDA forum user brk3, is not just better than the official app, it also has heaps of extra features that make viewing photos more fun.
Glimmr has a gorgeous interface, support for Flickr groups, the ability to comment on photos, and the ability to view EXIF and location data. Compared to the official Flickr app, Glimmr actually makes browsing new photos from your contacts fun and interesting. The app will even notify you when your contacts upload new photos. The only downside to Glimmr is that it's definitely a just a browser -- it can't upload photos or organise your albums. Although, when you authorise the app for the first time you use it, you do grant Glimmr permissions to upload photos, so those features are probably coming soon.
The app is free at Google Play, although there is a paid "pro" version that strips out the occasional ad and allows you to set images you see in the app as your phone's wallpaper.