Program for changing the aspect ratio of videos?

[ssdoom]

I use AVIdemux (avidemux.org). You can pass through without re-encoding. I choose mp4 or mkv output, then click "configure" and force the aspect ratio I want. I also use it to re-encode audio without re-encoding video. Sometimes videos will only have audio on one side, so I re-encode to mono to get sound in both sides. I also use it to chop Full length movies in to "scenes" with no transcoding.

For transcoding video I use Handbrake.