Are there any convenient sources of color palettes to use in ArcGIS software while producing maps?
The built-in styles also contain lots of professionally selected color palettes and ramps that you could repurpose for other needs. To look at the styles without having to dink around in ArcMap, you can view them in PDF form
You can download the ColorBrewer styles for use in ArcMap through the ArcScripts site
Late answer, but it could be useful. I developed an online tool to extract main colors from an image, an visualize their relations and proportions on a (HSL) color wheel :
It could be used to examine a map, considered pleasant, and retrieve its colors.
Another tool, to create gradient palettes :
Have you seen the map templates?
It is not only for ArcGIS, but you can use colorbrewer for that.
http://kuler.adobe.com/ is another source of predifined color palettes.