Web.py reminds me greatly of Sinatra http://www.sinatrarb.com/, a minimalistic very agile ruby web framework.

These sort of frameworks are ideal for developing APIs or stand alone (RESTFUL) services, anything that does not rely heavily on forms or a proper UI experience. Sinatra for example can only have one controller, so any app you develop will naturally do one thing (and do it well).

They're also perfect for prototyping ideas!