We use them appended to our error logs to be able to filter out logs by sections of the code; for example #userSignupFlow #asyncTask, etc.
I also use hashtags on Twitter to find things happening at an event where I am (conference, talk, etc.), and post pictures that I take etc/quotes from the speakers, etc.
On Instagram, I use hashtags in my searches to find pictures of cities where I have lived/visited that I miss; my girlfriend is an artist, and does public art, and uses them in a similar way to find people posting pictures of her works.
