This is a useful, although it could be designed nicer, tool for seeing the percentage of users viewing web pages at a given resolution (illustrated by the coloured areas):

browsersize.googlelabs

Further information on the development of the tool can be found here.

This is a great little tool for generating files full of randomised GUIDs... In Dave's words: "think index colums of databases and record IDs for user lists etc"

http://davewhite.net/guidgen/

How cool is this? new.myfonts.com/WhatTheFont/ 

If you've ever been wondering what a certain font is you can upload an image of it at this URL and it comes back with some guesses. I tried it with an image of some Rotis Sans Serif characters at 25pixels height (25% of the recommended height of 100pixels) and it was spot on!

Loving it!

Now this is good: pxtoem.com 

I found the link to it on Dave White's blog and its pretty damn groovy. It is hailed as “px to em conversion made simple”. Type a base font size in pixels, and the tool will produce a complete pixel to em conversion table, making elastic Web design much easier to produce.

I like those apples :o)

Just remembered this - a site allowing you to generate random Lipsum (Latin) for use when laying out where text is going to go.

www.lipsum.com

Scroll down a little and look to the right to see the generation tool. 

This is a little online tool by Adobe that I've recently sarted using. Its pretty useful for picking out sets of complimentary colours and generating swatches:

kuler.adobe.com

You can also see sets created by other designers.