Fbootstrapp is a toolkit designed to kickstart development of facebook iframe apps in both relevant sizes. It includes base CSS and HTML for typography, forms, buttons, tables, grids, navigation, and more, styled in the typical facebook look and feel.

I did a bunch of research on this lately and it was indeed hard to find any comparisons.

A playground tinkertool for rapid web iteration

Preboot is a super awesome pack of mixins and variables to be used in conjunction with LESS, a CSS preprocessor for faster and easier web development.

An easy to use, powerful, and flexible framework for building prototypes and production code on any kind of device. Start here, build everywhere.