Web 2.0 modules to get your site rockin'

I would be really interested in seeing demonstrations and applications of some really cool Web 2.0 (AJAX/AHAH etc) modules that can be used to funk up a site. It should go beyond simply enabling a module - I want to see innovative uses for them and understand how to replicate them on my own sites.

Examples could be anything from a simple jQuery hack to a full blown cross-site integration tool.