Not sure if anyone else had ever noticed this, but last night when I was in the Colour Picker while working in Graphic Converter, I accidentally clicked on the little magnifying glass along the top, which then gave me an enlarged magnifying glass that allowed me to grab a colour from anywhere on my screen, regardless of what application the colour was in. And, as the Colour Picker is the same throughout most applications, I would assume that this would carry over as well.
My only wish is that Apple would build hex colour codes into the colour picker (like in Photoshop), to make it easier to work on web design.
[robg adds: I honestly didn't think to click on the magnifying glass until reading this hint ... so while many of you may consider it obvious, hopefully it's news to someone other than me!]
Mac OS X Hints
http://hints.macworld.com/article.php?story=2003050811333816