I guess a lot of it comes down to common sense. Personally I tend to follow (so I've been told) - the Creative Commons act. If I come across something on the net that I wish to use in my own work, be it an idea, a piece of code, I generally tend to completely reshape it to a point where it would be almost unrecognizable to the original. Basically I will mold & learn from it. This is comparable to doing a legitimate citation, for say, a UNI assignment. You read a paragraph of text, learn from it, then in your own words write something similar.

Of course you can't go around doing this to everything. Especially if you stand to make a profit from it. Remodeling something with a patent pending on it will surely land you with a lawsuit. But one things is for sure, I always credit people, and if I ever made some considerable money from a project that involved the use of another persons intellectual property, I would be in contact with them to discuss business.