XRAY - This can bring up the box model and information of any element on the page. It shows dimensions, positioning, paddings, margins etc. Very useful for quickly determining the aspects of a page design.
View Cookies - Does exactly what it says on the tin, shows any cookies stored by your browser.
SpriteMe - Parses your HTML + CSS and suggests a list of images that you should have as sprites. Also able to generate and export a sprite for you. Good for making a website load more efficiently.
Color List - Analyses your page and detects all foreground and background colours used. Shows a swatch of colours, RGB and hex values.
Design Grid - Pops up a toolbar when you can enable / configure / disable a grid, rules, unit counters, and a crosshair to show your cursors current coordinates (x + y).
TinyURL - Creates a minified URL of the page you are currently viewing.
Please feel free to add anymore useful bookmarklets to the comments!