Aneesoft AVI Converter allows you to convert files from and to the AVI format. The application uses a welcome screen that asks you whether you want to convert to AVI or from AVI, and it launches a different converter for each of those tasks. They look the same, but they vary in the output formats that they support. For example, if you use the "convert to AVI" option, you will only be able to convert files to AVI, DivX and XviD, whereas if you choose the other option, you will be able to convert your AVI files into lots of different formats, including MPEG-4, H.264, MOV, FLV, 3GP, among others.
The graphical user interface has a nice design and there are a couple of extras in both converters. There is a video editor that allows you to crop and trim your video, and to add effects and watermarks. There is also a video player that lets you play your files before you convert them.
I tested this app with both an MKV file and an AVI file, and both were opened just fine and converted well enough.