Git has something called submodule support. This allows you to specify one or more other git repositories within another – a bit like svn:externals (except trickier, but more powerful of course :).
The git user manual describes submodules but it took me a while to figure it out, so I’m hoping these examples will help others (and me again when I forget and find my own page when googling about it :)
These examples deal with your_project and the project you’ll be adding as a submodule, other_project