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.
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.
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.
AudioSlicer is a Cocoa GUI application that finds all silences in an audio file and allows you to split it into several smaller audio files and to name/tag them properly. While most other tools split automatically according to certain criteria, AudioSlicer shows you all silences within a certain range of duration. You can then listen and decide whether you want to split there. The splitting is done without loss; there is no decoding and re-encoding of audio data.
TriTag helps you to get a clear MP3 collections by either creating a new filename out of the ID3 tag or filling the metatags using the information found in the filename using a user defined pattern -- easy due to a pattern builder. The first mode also allows to create a unique directory structure (artist/album/) similar to the one generated by iTunes "Consolidate library".