Anyone interested in participating or developing applications for the social side of computing could do worse than visit ProgrammableWeb.
Just as it sounds, this site is dedicated to developing to the web as a platform, and is especially instructive in the craft of mashups. Moreover, it reveals in each of the sites it tracks which APIs have been used, and even has a section entitled "How To Make Your Own Web Mashup" in which it guides users at all levels of expertise in how to make the basic decisions about where to start, and from there which APIs will be needed. It also of course provides a list of APIs to guide you as to what's available, with comments as to their use and, of course, where to get them.
I've blogged on the extremely neat personal mapping application Platial before, and a significant element of ProgrammableWeb indeed focuses on maps based on Google's APIs, among others.
An especially timely example is mapgasprices, which does exactly that, soliciting local input from users on the prices they find at the stations in their neighbourhoods. It's early days yet, but I'd say this site's usefulness can only grow like a snowball if the current price hikes persist.
ProgrammableWeb has attracted some publicity and its core comprises a blog and three dashboards, Home, Mashups and APIs, which are updated daily.
Quite apart from being an extremely useful developer's tool, for anyone simply wanting to keep on top of the state of this nascent but rapidly advancing art, Seattle-based site operator John Musser is doing a magnificent job.