Don't be fooled by its simplicity since as mentioned it has built in features that most other similar application come with nonetheless.
Few of the main features...
*. Elegant and simple GUI.
*. Has a Library browser.
*. Search for files within the Library.
*. Amazon album cover art support.
*. ID3 Tag editing (version 2) ability.
*. Automatic lyrics fetching.
*. Switch between compact and full views.
*. Sys tray icon + notification ability (those pop-ups which display currently playing file duration, etc)... are just a few to mention.
Although I don't think it's in active development these days but seriously the application itself downloading size is about 192Kb and it is well known for its ability to use less system resources (RAM) which is very important for dudes like me who has old GNU/Linux Laptop or PCs :).
If you use Ubuntu 11.04 Natty Narwhal, 10.10 or 10.04 versions then you can install Gimmix 0.5.7.1 by using the below command in your Terminal.
sudo apt-get install gimmix