I was actually supposed to release this project a couple of weeks ago, but work interfered (it always seems to do so). Anyhow here is AtomCart. AtomCart is basicly a very lightweight shopping cart in its purest form (hence the name), built in javascript as a jQuery plugin. It does nothing else except hold a very lightweight interface shopping cart, that supports adding/removing cart items and basic sum calculations. Dead simple and very easy to implement.
AtomCart is hosted on Github, so you can find it here: https://github.com/marteinn/AtomCart