Although I mostly use "heavy" frameworks, micro-frameworks are more than just toys. Well, I have to admit that I do like playing with, maybe because of the near-instant gratification while still keeping my Model-View-Controller (MVC) goodness.
Micro-frameworks are perfect for mini-applications or even single-purpose scripts. I've chosen one from each major open-source language community. I was looking mostly for small frameworks that stuff as much as possible in each file, like all the controllers in one file.
But, I am also looking for micro-frameworks that have a short learning curve and with which I can easily maintain the afore-mentioned mini-applications.
Here are some of the ones I like best so far.