These are some of our favorite client applications. There are some others out there and you can refer to the GeoServer client page for more information.
Udig Udig is a desktop client that supports OGC WMS and WFS requests. It is built off of the same library as GeoServer: GeoTools.
GVSig GVSig is a desktop client that supports OGC WMS and WFS requests, similar to Udig.