No, there isn't.

I get what you're asking. Maybe there's a secret website listing places that are too lazy to put up listings on the apartment listing websites. However, if there were such a place, it would just be another one of those apartment listing websites.

Like, Craigslist is a website that connects apartment buildings that don't post on MLS - but it's well-known enough that it's just part of your "places I already know about."

There are other websites like Zillow and ApartmentList, but I don't think you're just looking for another version of MLS/Apartments.com/CL. I think you're hoping for a magic list of "buildings that never list their apartments and probably have great vacancies I could get." If such a website existed, we'd all use it and then it wouldn't be the site you're looking for.

Your other option is to talk to a real estate broker. They have their own listings of apartments that landlords are looking to rent. In my experience I’ve already seen 90% of them by searching on my own (since brokers will also post the units on craigslist/zillow/facebook). Good luck!

I found my last two Cambridge apartments through a realtor and neither of them were posted elsewhere. That being said that was quite a long while ago so things may have changed. But at least you avoid scams on CL and other sites by going directly through a broker.

I do think Hotpads.com is the most exhaustive listings aggregator and it has an interface that is easier to filter and scan new listings vs the ones that you’ve viewed before. That said, there are plenty of small landlords in Cambridge who just list apartments for rent via signage in front of the building and you do learn a lot by walking through your desired neighborhoods!

Facebook is a good tool with multiple different groups you can look at, apartments.com, a realtor, or the housing subreddit