From Kevin Tom at the Official Google Blog:
Users and developers alike have been clamoring to know when they can develop themes for the iGoogle homepage, and we're happy to say that today is the day! Whether you like outer space, cartoons, dogs, or anything else, you can now create your own theme and help personalize iGoogle for millions of people.
The Themes API lets you customize many portions of the iGoogle page. Your theme can also update the page's design based on variables, such as the time of day or location. This makes it easy to create a narrative that unfolds throughout the day, a landscape that changes as the sun rises and sets, or an abstract image that becomes more complex.
Also, when did the Google Toolbar become awesome? I love how easy it is to create custom search engines from web pages, which conveniently become buttons that link to those pages -- way more efficient and cool-looking than traditional foldered bookmarks, and even better than most custom-built toolbars. (I run it on Flock, since I use Firefox 3 Beta, which is incompatible with the toolbar. But Flock + Zotero + Google Toolbar is one of the most sophisticated browsing systems possible.)