Learn how to use ffmpeg. It's really not very hard to do simple transcoding and there are lots of examples in the documentation and online. It can be as simple as
Code:
$ ffmpeg -i input.mp4 output.avi
Using some web based transcoding thing will require you to upload what you want to transcode and then download it again which seems like it could be potentially very time consuming.
I've the easiest way to get a copy of ffmpeg that is compiled with support for what I need is to use the static builds linked from
https://www.ffmpeg.org/download.html#build-linux