Okay guys, I've thought this through pretty good and I don't see any reason it shouldn't work. I haven't used it myself because I haven't had the need but I figure it might be able to help someone...
The idea is to stay in a house that is for sale or for rent. There's two ways I've come up with that could work.
First step is to of course find a house that is listed for rent or listed for sale. Use the Internet to find your city's real Estate associations site or use Zillow or one of the other big sites.
Next is to get an appointment. A lot of agents will use a call center to set up appointments. I would say about half. You won't have the codes/credentials needed to make this work so these ones wI'll have to be skipped. The thing is, you also don't have the login credentials to log into your city's associations website to see the "agent section" of the listing. So, you won't know which listings are using call centers for appointments and which ones are not. When you call the realtor of the listing you want to try to get inside of, just hang up if they say "call the call center" to get an appointment. Or you could try to bluff and say something like "they said you didn't provide showing instructions so I'm calling you" but this is probably not a good idea as they will probably call the showing company after they talk to you.
I know I'm explaining this kind of backwards but you want to find agents that are taking appointments themselves. It costs $50 or so per listing to use an appointment setting call center and some agents don't want to pay for it because they are just starting off or whatever. Try to look at agent profiles and find agents with a small number of listings that are not too expensive. It's almost guaranteed big shot agents selling expensive homes are going to use the service.
When you call, you're going to say something like "hi, my name is Bob with junkbox realty and I'd like to show 123 ghetto lane at 3 o'clock today. They are going to say okay, what is your public ID? A public ID is kind of a login that Realtor use to make sure other Realtors are really realtors. They can type it in the system and it'll show the Realtors credential. The public ID can be anything but people usually just make it their name or their first initial and last name. There's no way that I know of to look up public id's without a realtor login so you'll just have to make one up. Thing is, a lot of the time, Realtors won't check the ID. I would guess maybe half of the time even more. Sometimes they won't even ask. Just sound professional.
If they do ask, just hang up or you can again try to bluff. Although you can't look up public IDs, you can look up license numbers. Find the governing real Estate body for your city (this will be different than the site that shows listings) and you should be able to search for agents. Search for common names or whatever and find an active license and the ID associated with it. After saying look dammit, my public id is farfanugen dammit, it was my cats name.... why don't you just look me up with my agent code, it's "666666"?
The only thing I'm not sure about is if they'll be able to see that your public ID is not what you said it was... I don't know because I never check. I don't even know how to. This trick would definitely work on me. It's a lot of trial and error work but it shouldn't take too many calls to get an access code.
Another thing that can stop you is if they are using a "supra" box. It's an electronic system and you won't be able to get into those either. Again, they are expensive, so your best bet is to try this on lower priced houses. You need to get one with a combo. Also, of course don't go to a house that's not vacant.
Another little trick is, it shouldn't be too hard to guess the code on boxes that look like this:
So, although the box will let you set the code to whatever 4 digits you want, it won't let you set the order. At least all the ones I've seen that look like that wont. So if you want the code to be 7654, 4567 and 6547 etc etc will also work. And you can't set the same number twice so it shouldn't take too many guesses to get it open if you can find a box like that.
I'm sorry if this was confusing and I know that this whole method has kind of a medium low chance of working. However, it seems to be a good enough chance to make it worth it. Especially considering you'll be able to stay at the house for as long as it's on the market. Plus most houses will have electricity and water so sleeping with the AC on and taking a hot shower is probably worth it. Just be weary of nosy neighbors... they'll probably get suspicious seeing someone enter the house next door every night. Showing hours usually end around 9-10 pm so it's probably safe to go in after that. And of course, try to leave early in the morning to be on the safe side.
Edit: actually I believe you can set as many numbers as you want on those little boxes. Most people only do 4 or 5 though.