For over two years I have been using icons and color pallets from Tango Desktop Project. Most of my projects today contains both icons and colors from Tango Project. With a pre-defined pallet it is easier to choose colors, now I have only to choose from 27 colors instead of 16.7 million, and with the icons following same pallet it gives quite nice finish.
From time to time I have even changed default Windows Icons with Tango Icons. Also have used Tango theme for Windows XP, since I stopped using Windows Theme a while back, I have used Tango Pallet for creating color schemes for Windows Classic theme.