Omega Centauri Software
Listen to audio through your phone's earpiece when you earphones aren't handy.
Download free audio books from the LibriVox collection (need an audio player)
Completely change screen colors (red on black night vision mode, set gamma, etc)
Change screen colors (red, sepia, BW, set gamma, etc.) for fun and night vision.
Move many unmovable apps to SD on devices that support moving apps. Needs root.
Send reduced resolution photos by email/text/etc with minimum number of taps.