How can I repair a broken AVI file?

Here are some programs that might help:

• VLC should offer you to fix a broken AVI file's index when you open the file.
• DivFix++
• AviFiXP (only runs on Windows)

There's an extensive tutorial on repairing broken AVI files here: Repair broken or corrupt AVI files - AfterDawn: Guides

You can also try rebuilding the container by doing a bitstream copy with FFmpeg:

ffmpeg -i input.avi -c copy output.avi

I've had some luck in the past with a lot of broken/incomplete AVIs using DivFix, but not sure if it works well on anything later than Windows XP.

A newer version is DivFix++:

DivXFix++ is designed to repair broken AVI file streams by rebuilding index part of file. This is very useful when trying to preview movies which has no index part, like some files are currently downloading from ed2k or bittorent networks. DivFix++ is complete rewrite of "DivFix" program due it's bugs and low performance.

VirtualDub can do pretty amazing things with broken AVI files. Just open the file in VirtualDub, let it cook, and then if it's opened it successfully, re-save it as a new AVI file. Usually, you don't even need to change the compression settings.