Bookmarklets are 256 character or less javascript applications that you store in your browser's bookmarks or favorites folder. They are basically little utilities or extensions for your browser. More info at www.bookmarklets.com
The site www.bookmarklets.com is divided into sections for Page Data, Windowing, Navigation, and Design. Here are my first-glance recommendations, for what they're worth
Design
216 standard colors - Not a file - a js generated page with the colors
RGB to Hex/Hex to RGB - Conversion tools
Basic ISO Lation characters - é and whatnöt
Edit HTML of page - you've got to see this one...
also: Find and Replace in SourceWindowing
Resize window to 640x480 - Faster and more reliable than the control panel or quickrez.exe?
Navigation
Find Any in Page - Allows multiple keywords for Netscape search function
Search
Many bookmarklets are available for different search engines - saves you that wait time for the first screen
Page Data
Edit copy of selection - Select some text, choose this and a new window opens with the text in a form, with no HTML formatting.
Number of words in selection - Select some text for this info