I build a fairly sophisticated project last year on top of TBNL (now Hunchentoot) and some other libraries (CL-WHO and CLSQL being the main ones). I don't think this set of libraries quite counts as a framework in the Rails/Pylons tradition - it's more like web.py[0]. It's worth noting that reddit used the same tool set before they rewrote it in Python. I can recommend it without hesitation, if you're looking for tools that stay out of your way.

[0]I have a strong suspicion that web.py was influenced by TBNL.