Google toolbar
I have been playing around with Google’s “new” toolbar buttons.
Quite interesting stuff, even though the current version has quite limited features. Google’s so-called toolbar buttons are basically XML files that contain a search engine URL (both GET and POSTS http or https requests are supported), an embedded base54 encoded icon and some other optional stuff, like an RSS feed URL.
Buttons must be hosted on some website and can be added via a special URL:
http://toolbar.google.com/buttons/add?url=http://myhost.com/mybutton.xml
Of course, you can submit your button to a public gallery hosted by google so that other users can find them.
I have written a few buttons to do various searches on flickr (photos, tags, groups, discussions and members). Please check them out at my single-paged google page 
I have found that the current version mainly lacks of three interesting features:
- You cannot add locally stored buttons
- You cannot have more than one option for the search engines (and its value must be used directly in the URL)
- You cannot access the option via a context menu - you will have to change it from the google toolbar settings
Nevertheless, it’s a quite interesting feature (even though Firefox’s bookmark shortcuts might come in handier).