Besides those mentioned.
X - Cleans your cache,cookies etc at one shot
Cookie button - Quick way to set cookie permissions per site
Cookie culler - quick access to cookies for removal.
IEview - adds a right click to open a link with IE
All in one gestures - Mouse gestures, intergretes with TBE , so you can set mouse gestures for unclose tab.
Prefbutton/toolbar enhancments - Add options to turn on off java,js,referrer,proxies etc with a single click
Pastequote- For posting on forums such as this! Adds
to pasted stuff from the clipboard
Context search - adds your mycroft search plugins so you can highlight right click and search with any one of them
Smartsearch - Same as context search except it uses the bookmark searches
Conquery - like the above 2, but more powerful , it can automatically use parimeters like the url, the domain name of the site you are on etc as parimeters to send to the search form. Eg you can check a webpage for html validity, look up it's domain records etc.
Searchbutton - controls whether searches using the search toolbar open in a new tab or on the same tab.
And many many more.....