The buzz over browser based applications, called weblications by some, is growing. The responsiveness of relatively feature rich applications like GMail is inspiring. The magic that goes on is really quite straightforward in principle and the dictionary lookup made by this guy is a nice example of the simplicity.
Source is included.

My reservations with weblications is the same as with all previous "rich internet app" frameworks like Flash and similar technologies, that it invalides the meaning of the URL space. I like REST. But even from that perspecitve, the structured approach to weblications inherent in the use of the XmlHTTPRequest with standard dynamic html, since it is at least possible to reverse engineer the wire format used by XmlHTTPRequest, like people have done for GMail. The killer combination of Perl with LWP::UserAgent or WWW::Mechanize is not beat yet.