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Technology0 min ago
No best answer has yet been selected by Potatoman. Once a best answer has been selected, it will be shown here.
For more on marking an answer as the "Best Answer", please visit our FAQ.How harmless they are depends on your perspective.
If you don't mind keeping a history of every site you visit and when and what you do there and handing that history over to a marketing weasel then they're fine because that's what tracking cookies do.
However this sort of data collection and the directed marketing that comes from it largely funds the Internet.
See here for an overview
http://www.slate.com/id/2129656/