I found 3 great dashboard widgets for Mac OS X 10.4 and later.
In the past I always wound up looking up Hex and RGB color tables online. This widget is very nicely designed and makes looking up color values very, very easy. It gives you and converts between values in Hex, RGB, CMYK and even HSV. Get it here.
It takes the Spotlight calculator a few steps further (for those of you that don’t know this, if you’re running Mac OS and press command+space_bar or click on the magnifying glass on the top right corner of your screen, you can perform some basic calculations on the Spotlight bar). PEMDAS is a great scientific calculator with support for degrees/radians, trigonometric functions, converting hex, octal, decimal and binary, exponential/log functions, different notations, a history of your calculations, etc. In short, an easy to use scientific calculator with some nice features. Get it here.
3. iStat Nano
I love this widget. It gives you at-a-glance CPU usage, memory usage, hard drive info, battery stats (including total cycles and health), temperatures, bandwidth usage, fan speeds, uptime as well as your top 5 running processes. Get it here.