Android: Daydream mode is an underutilised feature on Android, but Dashclock makes it much more useful. By adding Dashclock or other Daydream-compatible widgets to your Daydream screen, you can have an always-on display of all the information you want.
Windows/Mac: Screenstagram is an attractive, cross-platform screensaver that displays the best of Instagram on your screen.
Windows: Many utilities, such as JkDefrag, jazz up their routine scans by presenting them as screensavers. Security Essentials Screensaver turns your computer idle time/screensaver activation into malware and virus scanning time.