A Kulerful tool from Adobe
The tool comes in a variety of guises including an online tool at kuler.adobe.com, Adobe Air desktop app, smartphone app and an extension in Photoshop, Illustrator & Fireworks.
The online and desktop app versions are the most complete with a few extra perks such as a trending tool called Pulse that gives an insight into colour usage by region and a tool to create your own palette from a base colour or image.
The Kuler incarnation which I find to be the most useful on an everyday basis has to be the Kuler extension included with Photoshop, Illustrator & Fireworks. As well as having the ability to search and filter palettes by popularity, rating etc, the Kuler extension allows the user to save palettes to the swatches panel. This helps ensure that once you have selected your palette, the use of those colours will be consistent across your design.