git remote add coworker git://path/to/coworkers/repo.git
git fetch coworker
git checkout --track coworker/foo
This will setup a local branch foo, tracking the remote branch coworker/foo. So when your co-worker has made some changes, you can easily pull them:
git checkout foo
git pull
Response to comments:
Cool :) And if I'd like to make my own changes to that branch, should I create a second local branch "bar" from "foo" and work there instead of directly on my "foo"?
You don't need to create a new branch, even though I recommend it. You might as well commit directly to foo and have your co-worker pull your branch. But that branch already exists and your branch foo need to be setup as an upstream branch to it:
git branch --set-upstream foo colin/foo
assuming colin is your repository (a remote to your co-workers repository) defined in similar way:
git remote add colin git://path/to/colins/repo.git