My preferred site is http://www.pictaculous.com/ because it offers a limited palette... Most of the sites give you huge palettes (19+) which is overwhelming. The biggest drawbacks to these sites is that you can't download, print or save the colors... I had to use the camera to get a picture of these colors because all that you can save is the color numbers which relate to various palette swatches such as Adobe's.
When you're making fabric choices it would be nice to have a color palette you love to use as a reference.