The DOPE Project hands out Narcan (naloxone) to people who need it in San Francisco.
Twitter for Harm Reduction
Twitter can be a very useful tool for sharing information and activism. If you're new to using it I've written a short guide to get you started.
I'd also recommend 'following' any of the accounts pictured below as all of them are people I've found have interesting things to say over the last few years.