I recommend the Better Privacy Firefox Add-On
I thought I worked hard to block unwanted cookies. bit this thing found between 140 and 160 flash-based cookies on the various machines I use. Scanning the list, I told it not to delete the Pandora cookies, and eat the rest. It's possible that I deleted something I'll miss, but I rather doubt it.
I really wish there were a best practices norm that people who use cookies have to make them human-readable, so you know what they say about you. Failing that, they should at least be expected to deploy a tool on their homepages that would explain to you what their cookies say about you.