I've come across a couple of links this week of lists and directories of Web 2.0-type web services / APIs.
First is the 'Web 2.0 APIs' list from John Musser at ProgrammableWeb, currently with around 60 services in alphabetical order (found via Richard MacManus > Josh Porter's Del.icio.us links).
The second is wiki directory of APIs WSFinder by Chris Law organized by category. Updates to Web Service Directory can be subscribed to via RSS. (found via Mr Scoble Snr > Brand Feld)