Free MKV Player supports the popular MKV file format, as well as AVI and MP4, but it doesn't handle high-memory MKV files well enough and it doesn't offer any standout features to make up for it. It also comes with some add-on programs during the installation.
When you install this program, it will try to load different add-on programs to your computer. These will take over your browser with toolbars, homepage switches, and more. Make sure you read the fine print and opt out of each. Though the program's name is Free MKV Player, it supports AVI and a few other video formats, as well. While it managed to play most files used for testing, when we played a big MKV file, it struggled to play it full-speed. If the file is more than 100MB, it will usually play it half-speed. The player, itself, consists of a single window and doesn't include any advanced menu options. It just includes the basics -- a timeline, volume control, as well as open video, play/pause, stop, and forward and backward buttons. While it plays videos in full-screen and lets you drag and drop files into the player for instant playback, it doesn't remember where you left off with a video if you close it.
With universal players like VLC available, there's not much room for video players that only focus on one or two formats. If you're looking for a basic MKV player that can also play MP4 and AVI files, then Free MKV Player would be enough. However, if you're looking for a more universal player with better playback quality of larger MKV files and more advanced options, you should look elsewhere.