bcr beats is a beat slicer that lets you cut up and play with an audio sample: you can trigger slices and loops with MIDI or the mouse and keyboard, set pan, volume, and sample rate values, and generally have lots of fun. The interface is based on the Behringer BCR2000 MIDI controller, which means lots of knobs. It can be used with or without a controller.
As a plugin API, Frei0r's main emphasis is on simplicity, to round up the most common video effects into simple filters, sources, and mixers that can be controlled by parameters. Frei0r plugins can be shared by many host applications, avoiding duplicate efforts by different projects. Frei0r is not meant as a competing standard to more ambitious efforts trying to satisfy the needs of many different applications and more complex effects, as it doesn't provides things like an extensive parameter mechanism or event handling.
irrKlang is a high level 2D and 3D cross platform sound engine and audio library that plays WAV, MP3, OGG, MOD, XM, IT, S3M, and other file formats. It can be used by C++ and all .NET languages (C#, Visual Basic.NET, etc.). It has all of the features common to low-level audio libraries as well as lots of useful features like a sophisticated streaming engine, extendable audio reading, single and multi-threading modes, 3D audio emulation for low-end hardware, a plugin system, multiple rolloff models, sound effects, and more. All of these can be accessed via an extremely simple API using an object oriented design.
kamix is a mixer for KDE and ALSA, with more features than kmix (or at least with more features than kmix had when kamix was started). It supports channel splitting, levels storing/restoring, sync with external ALSA events, correct handling of "enumerated" elements, and selective item hiding/showing.