In so many ways! Submit bug reports, review the documentation, participate on the forum, blog about PSPP.. all these things help drive the project forward, make people involved in the project stay excited, attract new faces and free up the developers to spend more of their time on building new features.
Cheers,
Patrick
http://patspam.com(Matt Trout gave a
nice talk about this at YAPC::EU 2008, although you might want to watch with the volume turned down if you have sensitive ears)