I have only just learnt how to type a hash symbol on Mac, as there isn't a default key for it. Due to an increase in Twitter usage over the last year, not just for me but for many, it seems silly that there isn't a key for it. Maybe there will be on the next generation of Macs. So anyway, you press alt + 3 to get a #.
But there's plenty of other symbols I occasionally use, such as hearts - on Facebook you just do <3 and it turns into a heart but this doesn't work on Twitter and the whole internet in general.