We have previously mentioned ColorBrewer, which is a great web tool for selecting color ramps for your maps. ColorBrewer 2.0 was recently released, and includes more information about the tool and its options. You can turn on a background hillshade layer, view semi-transparent colors, and filter your color ramps based on their suitability for printing and photocopying, and those that are colorblind safe.
The National Cancer Institute GIS has created the ColorTool plugin for ArcMap that you can download here, which makes it even easier to incorporate ColorBrewer color ramps into your MXDs!
