It doesn't have a huge list of features but comes with more than enough of 'em to let you easily manage a lot of audio files with ease + as mentioned in the title, it requires little system resources when comparing with some other ones as well.
It use the Gstreamer multimedia framework and if you have installed the proprietary codec pack in Ubuntu (I'm using it for testing this), then just download and install the player which is gonna take like below 0.7 MB concerning file size!.
Don't be fooled by the word "simple" because RhythmCat comes with a decent amount of useful features.
*. Excellent playlist support - Add multiple ones easily, export/import existing ones, removing, renaming and re-scanning (RhythmCat calls it "refreshing), etc.
*. Import music files by using either folders or individually.
*. Built in plug-in support so you can expand its features easily.
*. Has the ability to play file types such as Vorbis (also known as OGG), MP3, WMA, WAV, FLAC... as said it's based on Gstreamer so the list should be quite long :).
*. Excellent Equalizer with built in presets and save your own ones manually.
*. Play/Pause/Stop/Next... you know the usual "buttons".
*. Repeat or play files randomly.
*. "Mini mode": once activated the player switches itself into a very minimal looking GUI, may not always be useful but pretty cool.
*. System tray support (minimizing, etc).
*. Has 3 main themes.
*. Album-art display (but I don't think it's implemented actually because it just displayed the default art-work image all the time!).
*. And a "settings" window that lets you configure few advanced options of the player itself.
You can install RhythmCat in Ubuntu 11.04 Natty Narwhal by downloading the latest build for Ubuntu/Debian or other GNU/Linux distributions from this downloading page.
Although you can't search for files within it's main window, etc ... so may not be the best of its kind if you want to manage or find music quite easily as with Banshee, Rhythmbox, etc.
But if you want something that a lot faster and light on your system resources then RhythmCat is a pretty excellent one without a doubt!.