From a README: Travis CI is a hosted continuous integration service offered at no charge to open source projects hosted on GitHub. For full details see http://travis-ci.org.
You need a github account with a repository, but that's about it. You can use GitHub OAuth to sign into Travis CI, and following the directions there you can get it to run unit tests on every push to your repo.
The build and test process is configured with a .travis.yml file at the root of your repo. I've added one to
http://github.com/timburks/nu that builds and runs the Nu unit tests whenever I push to it, and I've also added a demo repository that shows how you can use Nu and "nuke test" to build and unit test your Objective-C code as a Mac framework:
http://github.com/timburks/NuTravisCI