Mainly directed at user6560's answer.
The answer on when using someone's hints or ideas becomes frowned upon is up to the professor on your class. The problem here is not whether or not you are using the hints in your write-ups, but the fact that you failed to disclose to us that you were seeking answers (or hints, or help, or whatever) to homework. My main problem is that it makes any action I take on your question a potential accessory to academic dishonesty, while keeping me in the dark about it (and, to some degree, makes me feel used). Had you disclosed ahead of time that these are homework problems you are working on for a course, I doubt it would have caused any problems.
Let me also say that I think you are misunderstanding the point of giving credit; it's not because we need or want the credit. You aren't insulting us by not giving credit. The problem is that you are potentially appropriating the work of others as your own. Even setting aside the issues of academic honesty, let me point out a few of the problems with this that you may not even have thought about:
Say I'm teaching a course, assign homework, including some problems that I know are challenging. My students turn in wonderful solutions, well-written, clear, often insightful, sometimes even clever, without having to ask me for help, essentially indicating that they are coming up with these solutions on their own. Not only will I get the impression that I have really good students (good for them), I will also get the impression that I am doing a truly wonderful job teaching the material (which may not be the case at all!), and that the material is easy for the students, so I can go faster, cover more advanced material, and spend some time on the more obscure but interesting bits since the students are getting the basics so well. If it turns out that the solutions are being obtained by asking for help from elsewhere, that the students are not getting the material as well as I think they are, then they are going to be very ill-served by a course in which I am going faster, with less detail, and not going over the basics as well as they need.
In a sense, the homework is not just for the students, it's also for me to gauge how things are going. If my students turn in work obtained from others without acknowledgement, then they are giving me a false impression of how things are going, which can be very bad for them in the long run.
In that same vein, I may construct exams that are too hard for the students, because I think the problem was so well understood given the results I was given. During the exam, they will not have the benefit of coming to the website to ask for questions, resulting in bad grades for the student. Not a good outcome at all.
So it's not about me expecting credit for the help, it's about you giving your professor an accurate picture of how things are going. In fact, if I were to give you an answer I would not require, request, or expect to be thanked by name in your write-up, though in order to abide by the plagiarism policy of your school you would need to mention that you obtained help/key ideas from this website (you could do so without mentioning me by name). I'm not sensitive because I'm annoyed at "missing a citation" (I don't even report citations of my work in journals when I file my annual work report). It's about being unhappy at being part of actions that will make someone else's job that much more difficult (a job with which I can sympathize, since I also have it).