Synergy is a tiny Cocoa application for Mac OS X 10.2 (and later) that puts three buttons to control iTunes in your menubar: previous track, next track, and play/pause. It also provides you with visual feedback about the currently playing tune and allows you to control iTunes via system-wide hot key combinations. These features provide a very fast way of skipping between songs when using the keyboard or the mouse, without having to switch to iTunes or access its Dock menu.
SynthTest is a somewhat simple application for OS X that allows you to pick a MIDI source, a MusicDevice AudioUnit, and an Effect or MusicEffect AudioUnit to play with. You can open an edit window for both the synth and effect. If they do not have a custom edit GUI, the generic editor view will be used. SynthTest has a reference implementation of preset support for Audio Units. It is primarily intended to be used by Audio Unit developers for testing purposes (the source is available), although it can be great fun to play with all by itself. It is complete enough to be used for live performances.
VLC media player is a multimedia player, framework, streamer, and encoder. It can play inputs like files, network streams, DVDs, audio CDs, Blu-Rays, capture devices, and screens. It can play most audio and video codecs and formats (MPEG 1/2/4, H264, VC-1, DivX, WMV, Vorbis, AC3, AAC, MKV, etc.), but can also be used to convert to different formats and/or send streams through the network.
skiTunes is a fast, extensible alternative to iTunes. It can use Apple Quicktime, VLC, MPlayer, LAME, and cdrecord. It differs from Apple's offering in that it is written in Cocoa and uses native widgets, as well as supporting formats iTunes doesn't, such as OGG and WMA. Using native widgets will allow it to automatically match the rest of the OS when a theme is applied, unlike iTunes, which requires hacking in order to make it match. It will also offer a skinnable "mini- mode".