My neigbors at the end of the street had this dog that they would constantly let roam all around up and down the street in this subdivision, in and out through a fire access gate to a relatively major street, off-leash, collarless and unsupervised. I live in a hot, desert area and summer's coming on, and he would often just sit on their doorstep panting without even a water bowl. A couple times I'd knock and get them to let him in, but one day recently he was out and they weren't even home (these houses have no yards, so it's not like he could've dug/slipped out, by the way.) So finally I called animal control and then enlisted the neighbors next to them in the next unit (who had much the same story as I did) took their dog to my house and had an Animal Control officer come and take him away. I feel pretty bad because he seemed to be an older dog and I'm not sure if he would've found a home, is why I'm asking this. Haven't heard a peep out of his former owners, incidentally. Should I have done this?

Of course this is only if they actually loved the dog!
Pets become part of the family for most (if not all) homes.
If the dog didn't even have a backyard then it seems that these people were not responsible to begin with in buying (probably stealing!) a dog they couldn't care for normally.
YES, you did the right thing. And in regards to 'Justsomejerk's' comment, the Animal shelter will ALWAYS try to find a good home for any pet before deciding on the only option left of putting the dog out of its misery.
Far as I'm concerned you were brought up right.
Sorry the other 'family' wasn't.
Judging from experience in these situations, they'll likely do the exact same thing again with another pet. They won't learn until they are charged. I do understand that we haven't heard their point of view, but it just sounds all too familiar in my view.
Don't worry about what you did for one moment. Whether it's a wake-up call for the owners to start treating their dog better or the chance of the dog being more fairly treated by others, you did the right thing.