SIMILE, the team behind the awesome Timeline and Timeplot embedded visualization tools are also working on making mashups where no API exists super easy. Potluck allows web pages to be screen scraped and introduces a simultaneous editing technique that has to be seen to be believed. There is also a nice transformation of the MIT Course catalog, though it uses lots other other tools from SIMILIE, Potluck is not used.
I don’t condone Screen scraping but sometimes sites are so slow to introduce an API and improve usability/functionality. This is where tools like Greasemonkey and Potluck become useful.