I can't remember who came up with the concept - I think it may have been squeemu or thebaconfat? - but obsession hats are hats that broadcast your obsessions to the world at large, so you can easily find someone with the same interests. I'm not quite sure how they were supposed to do this, but I imagined something that took aspects of the obsessions and combined them. For example: you have 'sherlock holmes', 'house m.d.', 'doctor who', 'life on mars' and 'top gear' on your interests list, so you would have a deerstalker, perched on top of which would be a little model of Gene Hunt's Ford Cortina with a tiny Jeremy Clarkson wielding a cane inside, being pursued by a Dalek. Brilliance.
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I can't remember who came up with the concept - I think it may have been