This page will help Commons users with firefogg, a Firefox addon which supports video encoding to OGV and WebM.
Using Firefogg for uploading videos
UploadWizard supports Firefogg and enables automatic upload of almost any video file regardless of source codec!
Using Firefogg for saving videos locally
Visit firefogg.org/make, select your video file, then save it to disk. The binaries (executable files) of the command line tools are also available as firefogg.org/nightly builds for users of other browsers.
In addition to encoding the file, firefogg allows other options to be set, including removing audio and changing the aspect ratio. Quality setting should be set on highest quality (ie. '10' in the advanced settings or '6Mbit connection' in the simple settings). All converting is done on your computer; the file is never uploaded on the firefogg website.
Firefogg builds on the ffmpeg library, which supports a wide range of codecs and containers. In general it should support most files you have.