+1  A: 

Check out this page to see all the video formats that Android supports for encoding and decoding.

http://developer.android.com/guide/appendix/media-formats.html

For encoding use FFmpeg or a service like encoding.com

Chris
What experience have you had with users downloading or streaming 3GPP or MPEG-4 on their Android device? And why would you choose one over the other? Encoding.com looks fantastic, but very pricey. I like the idea of FFmpeg.
Marc
3GP over progressive download has been reliable. MP4 is temperamental. It can work but the file has to be optimized for streaming and there's not much documentation on all the settings to make it work. I have done it with AAC audio, H.264 encoding.Another option for encoding that I forgot to mention is Quicktime 7 with Quicktime Pro. Costs $30 from Apple.
Chris