The Limits of XMLHttpRequest
Ajax via XMLHttpRquest (XHR) has been revolutionizing the web, driving the web2.0 movement. While there are complaints, the technique is quite effective. However, it does have a severe limitation in that the requests can only be made to a server within the same-origin of the currently loaded page. This was implemented as a security measure, but it's a questionable one to take (see Security Outlook).
Enter COWS Ajax and many Possibilities become available. Allowing a connection to foreign sites can lead to the creation of a new breed of powerful tools and favelets. Instead of each site owner making their own tools, now a single author can make and distribute a cool tool or service that is easily installed on countless sites with the simple addition of one or two lines of code. It's extremely easy to install for a site owner, can provide extremely powerful apps to the users, and can provide extremely good branding or revenue for a tool creator.