Dumpster Diving
In 1988 the US Supreme Court ruled that trash-picking is legal. But even before that, there are centuries-old precident laws going back to Jolly Old England that establish as a veritable "right" for scavengers to obtain & keep or sell anything they find in the trash.
A very few cities have nevertheless put restrictions on dumpster diving & local laws should be checked at the library (does no good to ask the police who won't know, won't look it up, but might say it's illegal rather than admit they don't know & don't care if its legal or not).
Exceptions are when dumpsters are locked, are inside gates, or posted no trespassing, or when there specific municiple restrictions which would regard it as theft to remove material from recycle bins as distinct from trash dumpsters of mixed refuse.
By & large when it's in the trash, it's fair game, whether you're the cops going through the trash looking for evidence without a search warrant (in most cases, they don't need a search warrant), a crazy nosy neighbor reading then posting on the web someone's discarded correspondence (no legal right to privacy if it is thrown in the trash, though some copyright protections to reproducing or "publishing" it might apply), a hungry homeless guy looking for pizza rinds, a craftsperson looking for weird junk to weld together into "art", a junk dealer looking for salable freebies, a major recycling company contracting with the city or county but not with whomever threw out the garbage, or a dumpster-diving hobbyist, such as a gardener, salvaging thrown-out potted plants or burst-open compost bags from behind a Walmart or Home Depot or from a compost heap at the cemetery.
The illegal part would be depositing your own trash in someone else's dumpster; circumventing a lock; or leaving a mess. When trash is on the curb or alley, there is not even a trespassing issue, but on business tarmacks or parking lots the issue of trespassing can become clouded, though if legal access is generally permitted for customers, so too it is for dumpster divers.
Garbage left on a property that does not permit general access is illegal to take -- that worn out couch on the curb you can take, but when it was still sitting on the front lawn getting rained on & moldy, it remained the homeowner's personal possession. There are also "intellectual property" issues. For instance, if I throw out a manuscript for an original short story & you find it, the manuscript is yours, but you can't publish it; or if you find a computer harddrive, it's yours, but the software on it might not be legally transferable; & so on.
Most dumpster diving is behind retail shops. The restriction (with exceptions) is usually a lock, not a law. No lock, no prohibition. Dumpster diving has become so common, though, that some cities feel the need to regulate diving, as sometimes guys with big trucks drive through alleys getting recyclables & whatnot, & sometimes bums leave nuisance calling-cards like all the black plastic bags ripped open & scattered about a parking area. A few states or municpalities are mostly concerned with dumpster diving only for the sake of taxing such microbusinesses which scrounge & sell enough stuff to make a living, who often fall underneath the radar of taxing authorities.
Habitual dumpster divers sometimes keep photocopies of articles on the Supreme Court ruling to give to irate shop owners who threaten to call the cops on dumpster divers. Politely informing them of the law, as long as it's not rudely expressed, & your reassurance that you'll be following the law by leaving no mess, usually shuts them up. If the owner doesn't care if it's legal or not but just wants you to go the hell away, they will have to post a no-trespassing sign or put a lock on their dumpster. As many do. Otherwise, if it's in the trash, you can have it.
Some businesses put stuff out back beside the dumpster with the expectationthat it will be hauled away by dumpster divers, & they're glad of it, it keeps them from having to pay for a bigger dumpster. But in some cases, as with bins of cardboard & paper which earn some stores a kick-back on its recycle value, or pallets of broken bricks or pavers which are picked up by the wholesaler for full credit & crushed to reuse, this type of material is not actually in the trash & one must be careful not to steal anything based on proximity to the trash, as this might indeed be an outright theft.
When in doubt, ask permission, but be prepared for disappointment. I once asked at Home Depot if I could have the broken bricks tossed out by the trash & was told yes I could take any of those I wanted. But at a later date I asked at the same store for the same sort of broken masonry bits, I was told I could not have them because they would be picked up by the manufacturer for credit to the store.
The only seriously bad side-effect of dumpster diving is the great many businesses that throw out credit card information. In 2002, Congress once again failed to pass a law requiring businesses to discard c.c. information safely by returning it to a bank or shredding it; Visa & Mastercard persistently lobbies against such laws, making one wonder what in the world the credit card companies have to gain by your numbers being stolen. This definitely puts consumers like you and I at risk for identity theft, sites like LifeLock.com go into further detail in this subject.Ý
The majority of illegally used c.c. numbers are obtained by dumpster diving. Yet people worry about emailing such information, which is still the minority source of stolen numbers (even hackers rely on info found in dumpsters. Dumpsters are the best places to find passwords). If your personal stats are in the trash, they can be legally taken by anyone who finds them, but their use of them will be illegal, which fact won't deter criminals.