3GP (or Third Generation Partnership Project) is one of the most common video formats capable of playback on standard mobile devices. These are typically low-to-mid grade phones, while SmartPhones tend to be capable of playing higher quality and higher frame rate MP4 and other video formats.
It is important for any video message designed for widespread distribution to also be converted into this format, as for the foreseeable future, it will be the dominant choice of video playback formats for Mobile Phones worldwide (though MP4 is slowly gaining dominance and adoption).
3GPP: 3rd Generation Partnership Project
The 3rd Generation Partnership Project (3GPP) is a collaboration agreement that was established in December 1998. The collaboration agreement brings together a number of telecommunications standards bodies which are known as “Organizational Partners”. The current Organizational Partners are ARIB, CCSA, ETSI, ATIS, TTA, and TTC. 
The establishment of 3GPP was formalized in December 1998 by the signing of the “The 3rd Generation Partnership Project Agreement”. 
- 3GPConverter Project: http://sourceforge.net/project/showfiles.php?group_id=192375
- AMR Audio Codec for 3GP video and Mobile audio files: http://www.penguin.cz/~utx/amr
- Ubuntu Forums section on how to compile ffmpeg with AMR support to ensure 3GP conversion/streaming capabilities: http://ubuntuforums.org/showthread.php?p=2960258
- 3GP ffmpeg-based Shell Converter Script: http://blog.mypapit.net/2007/04/ffmpeg-based-batch-flv-to-3gp-converter-scripts.html
- Ubuntu/Debian-distro guide to patching your ffmpeg for 3GP audio: http://blogger.rukker.org/2006/07/12/enable-mp3-and-amr-support-in-ffmpeg-ubuntudebian/
- Latest 3GP Music Videos resource: http://www.mv3gp.com/3gp_videos_latest93.html