BFG is based on WSGI and is from the same guys who brought you worked on the Repoze project. BFG only provides URL mapping, templating and security; the rest is up to you.

What I like about BFG (after a few minutes of perusal): views as callables (meaning you can define a view class), built-in permissions-based security (although rolling your own is fairly trivial), built-in event listeners (although, again, rolling your own observer pattern is trivial).

What I don’t like about BFG: ZCML configuration (XML, as opposed to Python), no Mako template support yet (although I’m sure I’d be OK with Chameleon).