It depends on your local codes/ordinances/bylaws/deeds if it's legal or not.
The only time I've gone looking in a dumpster was when I worked at my last job. Our building was next to an awning manufacturer and they had a "clean" dumpster where they only tossed fabrics that were left over from jobs. Very occassionaly you could even find a complete bolt of fabric. The stuff was too heavy for a regular sewing machine so I and most of the people I know couldn't do much with the stuff but it did make great drop cloths.
Now where I work now they have changed around the offices and shop several times and I've scored some stuff right before it was to go to our trash compactor. I got a nice heavy table I put in the garage. It has metal legs, a 2" thick top and the top is covered in formica so it cleans up easily.
I also got an office chair that was being tossed because it was the wrong color. Our company colors are gray and maroon but the chair is blue so they wanted to get rid of it. I have that in my garage also and sit in it all the time as I work at the work bench. Very comfortable and the price couldn't be beat.
I gave my father a metal rack that plastic bins clip to. He could use it for his spare parts he keeps for the repairs of Lionel trains he does.
I know plenty of people who have gained lawn mowers, tillers, snow blowers and similar items that were left by curbs to be taken away. Some only needed fresh gas or minor repairs to get them going again.
The best find I know of was 4 rims with tires, trim rings and center caps. They were in good condition and were the Rally style that was popular on the Corvettes back in the 70's and even used on the Chevelles and Novas. One of the guys I used to work with found them on the way to work stacked alongside a garbage pail with a note for the garbage truck to take them. They were dusty and the tires were dry rotted but the rims, trim rings and center caps cleaned right up with soap and water.