Google Toolbar has lots of nice things - one click and you're reading the news, shopping on Ebay, laughing at their collection of Stupid Videos. The toolbar gallery also has a selection of buttons that can help you while you're working: there's so many I can't list them all here. Here are some that look to be very handy, tho:
Dictionary.com
Thesaurus.com
Scholar.com
TrustyGuides.com
GoogleBooks.com