X11 is inherently a network-aware, cross-platform technology, meaning many people run X11.app with client apps running on remote hosts.
Mac OS X is UNIX, too, so most of those applications can be built and run directly on your Mac. Furthermore, some applications have created .app bundle launchers to provide better integration with OS X.
Do you have a favorite X11 client application? Do you want to help people test new versions of Xquartz with that app? Then please add it to this page! If possible, please provide a link to prebuilt binaries for OS X; if those aren't available, then link to the source code.