AudioUnits host and digital mixing application with recording functionality.
AU Lab is a free digital mixing application that hosts Audio Unit effects. It ships with the Apple Developer tools (for more info on that, read http://osx.iusethis.com/app/xcode:"here").
Note: the only way to retrieve AU Lab is by downloading Apple Developer tools.
AU Lab is in many ways similar to commercial apps, such as Rax by Audiofile-Engineering, Mainstage by Apple Inc. (included in their Logic Studio bundle). Still, it's free.