q Yahoo Developer Network (http://www.developer.yahoo.net)-Descriptions of the APIs exposed by the Yahoo services, including Maps, Flickr and Del.icio.us.
q Google Web APIs (http://www.google.com/apis)-Descriptions of the APIs exposed by Google services, including search and maps.
q Amazon Web Services (amazon.com/gp/browse.html/102-0240740-9496941?%5F encoding=UTF8&node=3435361)-Descriptions of the APIs exposed by Amazon, including the product catalog, search and queue service.
q eBay Developers Program (http://www.developer.ebay.com/rest/)-Descriptions of the APIs exposed by eBay.
q Programmable Web (http://www.programmableweb.com/)-Great resource site listing many of the available sites exposing APIs.
q Fiddler (fiddlertoolcom)-Excellent HTTP debugging tool.
q Curl (http://www.curl.haxx.se/)-Command-line tool for testing HTTP (and many, many other protocols).