To convert video pocket divx (http://divx.ppccool.com/
) works great. It can convert most stuff. If you have something it can't convert, like .flv, you can use super (http://www.erightsoft.com/SUPER.html
) to convert it to divx (or something else) and then use pocket divx to re-encode it.
As for mp3s it depends on what you want to do. If you want to just use a lower bitrate any encoder can do that (CDex, Exact Audio Copy, dB Poweramp, etc.). But you want to edit them down to use as ringtones you can get audacity (http://audacity.sourceforge.net/
) for free. I'm sure there is a guide on here somewhere on the best way to make ringtones (probably even specifies audacity).
Have fun and welcome
btw just to be clear these are applications for you computer, trying to compress video using a cell phone (no matter how "smart") would suck
"Oh, you're a loon aren't you? Where have you escaped from, I'll take you back." Lord Bravery