I am also a sucker for stray cats; especially if they are very friendly. When we moved into our apartment we found a black cat (which coincidentally freaked out my superstitious fiance lol). That little spoiled kitty got tuna for two days before I found out he belonged to the neighbors. He was not a leash, nor did he have tags. It's sad to think that one day he may get hit by a car or something but I have to leave that up to the owner.
I'm also just outside of Boston. Not sure where outside the city you are, but there are a handful of feral rescue groups north of the city that could help you out. I'll try to post on your actual thread once I have access!