@astrodust - Perhaps the @ sign? :-)

I actually just looked up hash tags on Wikipedia, and was surprised to see that the asterisk is starting to be used to give out "stars" and ratings to items: http://en.wikipedia.org/wiki/Tag_(metadata)#Star_tags

So I guess so far, microblogging has appropriated at least four symbols: @, #, $ and *. That leaves: !, %, ^, &, and ().