This is a nice tool for doing mockups called Balsamiq...

http://www.balsamiq.com/

Here's another great design tool...  ColorPix

Have you ever wanted to get the Hex code off that beautiful image... without having to open up Photoshop?

ColorPix is a really quick and easy tool for grabbing colors off images or any other application or web page.  The download is a simple .exe file and when you run it you get a little box that will pick up the color data of any item on your screen.  Just move your mouse around the screen and the box will display the pixel location, RGB, HEX, HSB, and CMYK values.  Click the "Magnifier" button to get a zoom-in of the area surrounding your mouse.

I must say that I love cool and simple tools.  

The following blog at Six Revisions has a great list of useful tools.

12 Useful Web Tools for Designers

FTP - File Transfer Protocol is a web developer's means of uploading files to a web server.

All web designers/developers must FTP at some point, but the tools we use can make or break the experience.  I have found Smart FTP has out done my expectations for an FTP application.  You can try it for 30 days, but even the  $36 price is well worth what you get... in my opinion.