Free solution for multiple-source audio mixing

I'm looking to do live broadcasts of video games, which require mixing together the game audio as well as microphone audio.

In the case of an actual game system being used, I have external hardware that can mix the console audio, and my microphone. However, in the case of a game being played on the computer itself, I have a problem.

When specifying the audio source, I can only choose 'computer audio', or my microphone.

I believe some means to accomplishing this are Rogue Amoeba's Audio Hijack Pro or Ambrosia's Wiretap Anywhere, but each of these have a cost associated that is a bit more than I'm able to pay at the moment (and in the case of Wiretap Anywhere, MUCH more than I'm able to pay).

Is there some sort of audio mixing software that will allow me to create a virtual device consisting of my computer audio (or a specific application(s') audio) and my microphone audio, as one input device, suitable for using in another application?

Search in Spotlight for "Audio Midi Setup." It looks like this:

Clicking the little "+" sign in the bottom-left corner and clicking "Create Aggregate Device" will allow you to combine various inputs and outputs on your system into a single one.