I selected a color picker for a style editor in Tabs Studio and here is a list of most interesting open source WPF dialog based color pickers that I found (not commercial and not drop-down color picker controls).
Windows common color dialog
First of all there is the common Windows color dialog. Providing consistent user experience since 1990:
Edit Colors dialog in Windows 7
Color selector dialogs in Windows 3.1
Simple WPF Color Picker by Sacha Barber
Sacha Barber wrote the CodeProject article about his WPF color picker and later Mark Treadwell did various enhancements on this code. Still color position is not preserved when switching swatches, colors range on swatches is limited and there are no additional controls to tune color other than transparency.
WPF Color Picker by Sacha Barber and Mark Treadwell
Color Picker Custom Control from Windows SDK
You can find it in the “c:\Program Files\Microsoft SDKs\Windows\v7.0\Samples\WPFSamples.zip” archive on your machine and read about it in the Uncommon Dialogs: Font Chooser & Color Picker Dialogs blog post. Functional, but requires more polishing for practical use.
WPF Color Picker Custom Control sample from Windows SDK
Color Picker from SharpDevelop
Open source SharpDevelop IDE contains a color picker with a nice conventional design.
Color Picker from SharpDevelop
ColorComb by Shawn A. Van Ness
Shawn A. Van Ness developed the ColorComb color-picker dialog with the ability to adjust stylus stroke properties. It is very easy to select a color you have in mind looking at the comb and use the brightness slider for fine tuning. Cells in the comb provide palette like qualities allowing to choose a distinctive collection of colors. Lack of HSV and RGB controls ensures less distraction when selecting a color.
ColorComb by Shawn A. Van Ness
ColoRotate
ColoRotate is an online service allowing you to select colors in 3D. It positions itself as the most intuitive way to work with colors “in a way that matches how our minds process color.” The site has an interactive demonstration showing how “our visual nerves register color in terms of the attributes of color: the amount of green-or-red; the amount of blue-or-yellow; and the brightness.”
ColoRotate browser
Colors dialog in Microsoft Office 2007
Colors dialog in Office 2007 has a design very similar to ColorComb plus it has comparison between a new and the current colors.
Colors dialog in Microsoft Office 2007
ColorComb simplified
To be a part of a XAML editor in Tabs Studio, I liked ColorComb most. I removed ink-specific settings from the dialog and added the current color for comparison.
ColorComb without ink-specific settings
If you want to use this ColorComb modification in your application, following is the download link: ColorPickerSampleSV.zip.
Tabs Studio Blog (organizing Visual Studio document tabs) 
Where can I get the sample code for Microsoft 2007 color dialog?
Comment by Tony — October 14, 2011 @ 12:21 pm
Tony,
I haven’t seen an open source color picker close to Microsoft Word’s color dialog.
Comment by Sergey Vlasov — October 14, 2011 @ 9:45 pm
I have seen one from Devcomponents.com. DotNetBar for both windows form and wpf. But that is not the case. One can develop his own using an example from devcomponents.com. I have recently developed a very good one better than Microsoft.
Comment by Tony — February 1, 2012 @ 4:43 pm
Recently I have modified the color comb to look exactly like that of Microsoft. The work will soon be available for everyone. For more info email me at tonielro@yahoo.com.
Comment by Tony — February 29, 2012 @ 4:03 pm