A few of my friends have recently asked me how to share code between their applications. Here's a screencast walking you through how I solve this problem. It works really well for me. I hope this is useful to you.
The git submodule documentation might also be useful to you if you're new to working with submodules. If that's a little hard to understand (I know it is for me), just try googling it. There's a lot of good resources on submodules.