For rooted devices only!
This app will let you conveniently switch out the emoji in your system. Currently available free emoji are Google (stock), Samsung, LG, and iOS. For a small charge, Twitter and Emoji One are also available.
Thanks to gonsa from XDA for providing the font files: http://forum.xda-developers.com/showthread.php?t=2618523
Busybox is recommended, and the app is untested without it.
2.0 (July 9):
More emoji sets, now fetched from a list online!
No more emoji packaged with the app, taking up less space
Bug fixes and improvements
1.6.3 (July 1):
Fix for HTC devices
More coming soon!