As for law I'm pretty sure it's illegal. If you get caught though, it's unlikely you'll be charged with anything. You might be banned from the premises. Unlikely doesn't mean impossible though, so still be careful.

I know a few people that have been into doing it and I have a few times. In general, the cumberland-androscoggin-york area is pretty good for dumpster diving (never done it elsewhere so it may or may not be good in other parts of the state.) A lot of places throw out food stuffs that technically expired that day or have a defect in their packaging. This is a lot safer to do in the winter when it comes to perishables.

Some stores even throw out their non-foodstuff "breakage," even if it's only a small defect or cut in the packaging. My sister has got some pretty nice things, like a computer monitor that had a hardly noticeable scratch on it and a relatively nice disposable cell phone that goes for $120- it had a cut in it's packaging, looking like someone started to cut it open to try to steal it.

The holy grail of maine dumpster diving, though, is dunkin donuts. Not because it's anything too good but it's pretty much a guaranteed success. At the end of every day, they throw out everything that wasn't sold, from donuts and muffins to bagels. Usually there'll be a few big trash bags that're filled up, which don't have any actual other trash in them. Make sure to get it right after close though, and make sure that you dont grab one of the bags from nights prior. A good way to tell is to scout out the dumpster before they close so you'll know which bags will be the old ones. I used to live right behind a dunkin donuts and wasn't exactly rich so I practically lived off of bagels, muffins and donuts for a few months. Come to think of it, I still have dozens of bagels in my freezer.

I've scored a ton of nice stuff from the dump as well, if you want to give that a shot. I can do basic soldering and can thus fix ~95% of the "broken" electronics people throw away. It's really pretty crazy how much easily fixable shit people throw out. Broke chain on a bike? chuck the whole thing. Tv wont turn on? chuck it. Broken laptop screen? chuck it. I've made thousansd of bucks fixing and selling broken shit people throw out or give away for free. For example, my current tv is a decent, great condition samsung that goes for a few hundred bucks. There's also usually things like tires, bikes and other electronics but be careful- a lot of dumps dont allow you to take anything with a lot of metal in it, like aluminium wheels or copper&aluminium-laden air conditioners. Sometimes your not supposed to take anything at all. It varies by dump. quite a few have an area to leave broken shit or working stuff that people just want to get rid of, which you can take stuff from without breaking any of the dump's rules.