This Ingredients to the Pylons Python Web Framework demonstrates nicely why all python web frameworks should be based on WSGI now (not nice to have in the future but needed now). RhubarbTart is another good example of the advantages of building on WSGI (and specifically both these use PythonPaste).
An alternative example is httpy — a sane and robust HTTP library for Python which does indeed look sane and have some neat features, but surely now is the wrong time to launch something that aims to compete with WSGI. In fact I suspect that httpy would fit easily under wsgi, in the process allowing some code to be dropped. Essentially all httpy needs is a wsgi coupler (in its own terminology).