One little advertised feature of GMail are "Related Pages" that show-up below "Sponsored Links". Not-only is gmail able to offer you advertisements that are related to what you are reading, but also offer you links to relevant regular web pages.
This has come extremely handy when reading and exchanging emails from technical mailing lists. I've found myself pasting some of those links into replies, because they were simply that relevant. Confirmation emails from online travel brokers are also fun to read: you're reading about a flight itinerary from Los Angeles to San Francisco and seeing offers for hotels at both places.
DHTML and Browser Support
On Mac OS X, GMail recommends you use FireFox as your browser. I've been testing GMail on Safari and it has been behaving nicely. Safari is quite standards-compliant when it comes to the various aspects of its DOM implementation. I need to further dig thru obscure features to understand why they're currently discouraging people from using Safari.
It is refreshing to see such a prominent company bet their user interface on DHTML. The W3C has come along way in defining standards required to transform DHTML from the ugly mess it was in the late nineties to the practical reality it is today. Browser developers and vendors are enabling us web application developers to harness this renaissance of standards compliance into compelling web-based applications.