Any pure-python WSGI server works: gunicorn, cherrypy's server, wsgiref/werkzeug.serve, probably Aspen.
I'm pretty certain mod_wsgi does not work with Pypy (it uses Python's embedding API, which Pypy does not implement), I have no idea about uWSGI although messages like this: http://lists.unbit.it/pipermail/uwsgi/2011-March/001641.html make me thing it should be possible.
As far as I know, while a Tornado (framework) application can run in a WSGI server via tornado.wsgi, Tornado's evented server does not support mounting WSGI application.
thadeus_venture was specifically asking about WSGI.
I'm pretty certain mod_wsgi does not work with Pypy (it uses Python's embedding API, which Pypy does not implement), I have no idea about uWSGI although messages like this: http://lists.unbit.it/pipermail/uwsgi/2011-March/001641.html make me thing it should be possible.