Note: This works smoothest in browsers other than Internet Explorer (of course).
Click the buttons then drag the dots on the color wheel or...
Paste/type hexadecimal color values (e.g. #123 or #123456) into the boxes and hit "Enter".
Years ago I coded a very basic webpage color picker. At the time, it was a moderately novel idea who's implementation was rather on the simple side. It worked well, but it's functionality was a bit limited, mostly due to my lack of in-depth knowledge of scripting languages.
As the idea languished, more and more color pickers showed up on the internet, most of which made mine (which used only "web-safe" colors) look like it was coded by a (lazy) junior high school student.
I spent a lot of time Googling to try and find the page again (I never did), but while searching, I found that several other people had already made (free-to-use) gradients that were better than the one I had planned to create from scratch.
After doing some comparison shopping I eventually settled upon Farbtastic. Though I've read (but don't know) that the developer isn't supporting it any more, it's received high marks in terms of accurate color redendering and is used by "players," Drupal and Wordpress, among others.