Client-side implementations are listed at the W3C XForms homepage. FormsPlayer and X-Smiles are best-known.
Chiba is an open source Java implementation of XForms. It should be pretty cool, but it's a typical open source project: poor documentation for starters.
The Orbeon Presentation Server supports XForms, is LGPL software, and is very professional (was commercial and closed source until recently). Unfortunately, they do not really separate the XForms implementation from their own pipeline stuff. I managed to get it working, but this is not trivial. We might be able to extract the XForms parts ourselves. Orbeon looks very promising to me.