You may try
isquint. It’s totally free, but I’m not sure if it could convert video to avi.
If you would convert video frequently on Mac, I recommend you an
all-in-on mac video converter.
Key Features.
1. Convert various format video(MOV/MP4/FLV/AVI/WMV/MPEG...) to each other.
2. Support HD video.
3. Customize video specs.
4. Video editing function, including crop black bar on screen, trim time length for a specific video clip, merge several video clips into a single file etc.
5.Support batch conversion and merging video clips into one file.
---