If you're wondering how Google pulled off their impressive Gmail user-interface, or why their mapping site is so freaking cool, then look no further than "AJAX".
In February, XML.com's Drew McLellan wrote an excellent overview of Ajax called "Very Dynamic Web Interfaces". His article, probably more than any other, introduced the tenets of Ajax to a wide audience.
Better still, the folks at ModernMethod have introduced SAJAX, one of the best compact libraries I've seen for simplifying an AJAX implementation. If you write in PHP, Perl, Python, or Ruby, SAJAX is a great jumpstart on your first dynamic web app.
Check it out: SAJAX.