Hash tags (#) are a way of aggregating posts on twitter or facebook or other social software applications. They are driven by the community and have been used to great effect recently by the G2o protesters in London (#g20) and for the Amazon censorship of gay literature fiasco (#Amazonfail). I would imagine that they should only be used in emergencies; so use with caution because you never know when you may need your freedom of expression!
Here is a site that aggregates Twitter hash tags (link).
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