The first thing I did was add searching the Apex documention to the Omnibox (otherwise known as the location bar). Our documentation search engine uses XML to cache potential search results, and this extension works directly against those files to generate results, as opposed to using the site's normal search. This mechanism works equally well for Visualforce or API documentation.
Another option to do searching like this is Cloud Developer's Search Engine, which provides a Google search into all the main Force.com content sites.
If you need the more Swiss Army version of such convenience, check out Jeff Douglas' Force.com Utility Belt, which also allows you to search not only against the documentations, but the boards, blogs and more as well … and even offers the latest blog post towards the bottom of the extension window. Jeff also rolled his handy ID converter into this extension, so you only need the utility belt to get that done.
Speaking of ID's, you might check out gokubi's recent ID Clipper extension, which gives you the ability to cleanly grab the ID of the record you're viewing. It's got the ability to flip between 15 and 18 lengthed ID's, and can grab them off Visualforce pages as well.
Another handy one is Force.com LOGINS (the caps are in the title, I'm not yelling at you) which helps manage login information for multiple Force.com accounts.
If you don't see functionality you want here, you might check out the code.google.com site for creating new ones.