Twitter have released an open source HTML and CSS starter kit called Bootstrap. It provides defaults for front endy bits like typography, forms and buttons.
Better still, there's a Pinax theme based on it, making bootstrap
styling your forms as easy as
I can see this being a really handy starting point to get playground apps and prototypes up and looking reasonable without needing a frontender or designer. Well, until every other app looks like it and we get totally sick of it, obviously.
The GOV.UK Design System is a toolkit and a set of guidelines for creating sites and services for Her Majesty's Government which are clear and consistent and accessible.
MangoPay presents itself as "Payments for Marketplaces", but it's far from being just a payment provider...