I've used this tool (found through the Drupal forums) to create the favicon for several sites (including the wombat icon used here). It works like a charm! Upload your picture and it turns it into the 16x16 .ico file for you.
http://www.dynamicdrive.com/favicon/
Don't know what the favicon is? It's that little picture that appears in your address bar or by the link to a site in your bookmarks.
As stated on the tool's page, "The Colour Contrast Check Tool allows to specify a foreground and a background colour and determine if they provide enough of a contrast 'when viewed by someone having color deficits or when viewed on a black and white screen'[W3C]."
A neat little tool to show you the organization of content on your website. I'm not sure how practical it is, but it sure is fun to watch those nodes start building on each other and flying all over the place.
I'm horrible at matching colors unless it's black and white with varying shades of gray. Here the blues were chosen by "locking" the relation of the default colors to one another and sliding around the blue scale till I liked what I saw.
In any event, poor color schemers like me might find this tool useful:
http://wellstyled.com/tools/colorscheme2/index-en.html
It matches up various shades of the appropriate colors based on a few different styles. Use the arrow button on the very top right corner of the page to rotate the colors around. I probably won't change this place, but it's certainly nice for future sites...
Here's a quick little tool for SEO analysis. (Search Engine Optimization... if you care about that kind of thing.) This site is 'satisfactory,' which is good enough for me. 
An incredibly handy little tool to test the display of different fonts in your web browser. Do comparison tests and ensure the fonts you use are browser-safe: