Viber ( Android)
With audio and video calls and more features than most messaging services, Viber is the best option, but only if you can convince your friends to use it.
Celestial ( iOS)
Though it has some issues, Celestial is worthwhile for its relaxing, ambient sounds.
Fotonica ( iOS)
Fotonica is a never-ending running game that's worth it for the "Tron"-like visuals and ambient electronic audio.
Salient Eye ( Android)
As far as free theft-deterrent and motion-notification systems go, Salient Eye has the lock on simplicity.
FreeTone ( Android)
If you're looking for a spare number to deal with people from Craigslist or similar services, FreeTone does the job, but with the syncing problems and so many ads, there are better apps out there.
Pricing not available
Angry Birds Transformers ( iOS)
Angry Birds Transformers is a welcome departure from the slingshot approach found in previous Rovio games, and it's still very entertaining -- as long as you don't mind freemium timers.
QuickPics Photo Manager ( iOS)
With easy to use tagging features, QuickPics gives you a better way to search your photo library, but only if you're willing to put in the work.
Humin ( iOS)
With better ways to search your contact lists, and tons more information for each contact, Humin is an excellent replacement for your iOS dialer.
Glide ( Android)
If you're familiar with other video-chatting services, Glide offers more flexibility and features.
Bookviser Reader ( Windows Phone)
Despite its sluggishness, the Bookviser Reader for Windows Phone is a great way to get free classic literature on your Windows Phone.