Format Factory is very easy to use, although it does have rather an unintuitive interface. The file browser doesn't appear until you select the target format, which is a bit weird but as long as you know this first it's no problem. You can select individual files to convert, or complete folders, and also tweak some basic conversion settings.

Format Factory isn't particularly original but what makes it really useful is that it handles images as well as audio and video. Most converters like this only handle audio and video, and adding images to the mix gives it an edge, especially considering it's free. I also like the Audio Joiner in Format Factory, which allows you to take two separate clips, join them together and then convert them. It would have been nice to have had preset profiles for mobile conversions but you cant have it all in a free app sometimes.

Format Factory is an excellent conversion tool for anyone that converts between video, audio and images on a regular basis.
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Format Factory supports the following formats:

Audio: MP3, OGG, WMA, M4A and WAV.
Image: JPG, BMP, PNG, TIF and ICO
Video: MP4, AVI, 3GP, MPG and FLV
Recent changes in Format Factory:
Fixed some problems of "Fail to decode"
Fixed some decoder conflict
Fixed some problems of MKV conversion
Added some codecs to support more media file types
OS requirements for Format Factory:
OS: WinNT/XP/2003/Vista/7