It's a pretty incredible invention / discovery on Peter Parker's part, according to Wikipedia:
Shortly after getting his powers, Peter Parker develops a special synthetic polymer adhesive that has spider web-like properties, as well as wrist-worn launching devices.
Upon release, the webbing dries into an extremely tough, flexible, adhesive fiber. One account described a single strand as stronger than piano wire and it is perhaps as strong as real spider silk or Kevlar.
In Spider-Man: The Ultimate Guide, one strand of webbing is described to be strong enough to bind the Hulk and hold him prisoner, but only if the Hulk were to hold still and let the webbing sufficiently dry.
But has he ever sold it or profited from it in a financial way?
I'm sure the cash could help out himself and Aunt May, not that they're desperately poor, but that extra income would surely make things easier.