I was very excited to find a spellchecker built into the new version 3 toolbar for Google. It's perfect for checking text typed into forms on the web, making it indispenable for bloggers or anyone else who fill in a lot of web forms. Even better, I could use it to spellcheck form fields on some of our intranet applications.
A somewhat controversial enhancement is the AutoLink feature. If you click on AutoLink, it looks for ISBN numbers, street addresses, and package tracking numbers in the web page you're currently viewing and creates links that you can then click for further information. Click on an AutoLink address and Google will give you a map showing the location. If you're viewing a page with an ISBN number, click Autolink, and the number will now be an active link to Amazon. In particular, this last option made for some cranky people at Barnes and Noble who quickly created their OWN links on the ISBN numbers to keep people from being directed away from their site.
With the Barnes and Noble situation in mind, you can see why some people aren't crazy about the fact that Google essentially modifies the web page, or at least, changes how the web page is presented. It doesn't bother me...it seems to me that I should have that choice. But other people that I respect are concerned, so I'll give you a couple of another references for those discussions:
For more information on additional Google Toolbar features, see A New Version (Beta) of the Google Toolbar is Now Available, Still No Firefox, by Gary Price.