Merger is a free utility AU plug-in that combines two audio channels into one. This is typically useful for (e.g. vocoder) plug-ins that expect their two inputs on either side of a stereo pair.
In Logic 7 it is used as an insert effect on a mono or stereo track. The other input is selected from the 'side chain' menu.
NOTE: It doesn't work in Logic 6, presumably because of that host's Audio Unit implementation's limitations.