Aside from solutions manuals (which I never actually saw before), Chegg (used to be called Cramster) is quite popular. It provides step-by-step solutions for problems from a large variety of text books. If a physics class uses an online-based homework system such as Mastering Physics that only needs the final answer inserted, it can be a Godsend for certain students. I remember reading a thread a few years back where a student told their professor about this,
and ended up getting an IA themselves! Some people consider it cheating, but some don't. Generally speaking, if you use it simply to copy and paste your work into homework, then you will most likely screw yourself over come exam time. But if you take the time to understand how exactly the problem is solved, then this can be very helpful, especially if your professor isn't the best at explaining things. It can do a hell of a lot better job explaining than a textbook! If you are lucky enough to have a professor that allows cheat sheets on exams, then something like Chegg can help you do very well. Of course you'll have people debate whether it is ethical or not.
Unfortunately, after searching, I couldn't find the thread about the student who told their professor about it. I did find a very brief one from a few years ago about using the service. Now these members are pretty far along in their medical education journey, so it comes to show that you can use something like this and still be a perfectly good doctor.
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http://forums.studentdoctor.net/threads/has-anyone-used-cramster-com.760203/