One toolbar that's not included in Chris's Firefox overview is one released a few minutes ago.
Clusty's Toolbar for Firefox offers up what you've come to expect from search toolbars along with a context-sensitive searching option called "Clusty Clips." This feature allows the searcher to access definitions and encyclopedia entries (via Wikipedia) and run searches without having to click off of a web page.
Finally, Michael Abramowitz from Yahoo, offers up a Firefox/Yahoo extension on the Yahoo Search Blog.
This Year's Premier Digital Marketing Event is #CZLSF
ClickZ Live San Francisco (Aug 11-14) will bring together the industry's leading online marketing practitioners to deliver 4 days of educational sessions and training workshops. From Data-Driven Marketing to Social, Mobile, Display, Search and Email, the comprehensive agenda will help you maximize your marketing efforts and ROI. Register today!