Those projects that practice unit testing would surely welcome decent, well-written tests. In projects that have been around a while, there's often all manner of functionality that hasn't been tested. Given that debugging and testing can be a bit of drudge work, projects often greatly appreciate someone taking the effort to do the necessary but not terribly sexy work. (Add writing documentation to the list of things-to-do!)
We see it in Squeak all the time, for instance, where an excellent way for newcomers to get their feet wet is to write tests for something that doesn't already have tests.