Do free online collaborative solution manuals exist? I'm not a mathematician by training and a rarely come in contact with mathematicians. For this reason I find solution manuals to be incredibly useful - reading them allows me to see how experienced mathematicians solve problems and I often learn as much from the solutions as the original texts. Even if I can solve I problem myself, I usually have a read through the solution to check whether there is an alternative proof I haven't thought of.
Even though a lot of textbooks have solution manuals and a lot of these manuals end up on the web, I occasionally come across some, e.g. http://terrytao.wordpress.com/2011/01/24/an-introduction-to-measure-theory/, for which it seems that either no manual has been written or the author's has done a very good job of keeping it private (I'm not saying that this is a good or a bad thing - it is none of my business). While attempting to work through these texts I keep thinking that it would be great if there was some sort collaborative online manual where individual readers could post and discuss solutions for a particular textbook. My questions then are:


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*Has anyone ever heard of such a thing? If so, did it work? If it did, how does it work and if it didn't, why not?

*If there are any authors reading this - how would you feel about such a solution manual appearing on the web for your texts?

*Is it even legal to create such a document?
 A: There is a mathematical physics book called "The Road to Reality" by Roger Penrose for which a forum has been created, including discussion and solutions for exercises. You can find it here to see if this is something like what you had in mind
I have no idea about the legality of such a thing, although the creators of the previously mentioned forum received permission from the author of the book to create it.
I think something like this would be good to have for the purposes of self-study, although the impression I get after talking to professors (i.e. anecdotal evidence) is that authors want to encourage students to struggle through the problems first without having an easy resource to fall back on. (I think the reasoning for that is looking at an answer without thinking hard on the problem will spoil the "mathematical growth" associated with the exercise, and having solutions would be too tempting for some students.)
A: I've recently started a public repository with my solutions to the exercises from Concrete Mathematics.  Right now its really just for myself, but anyone is free to contribute their own solutions.
A: (a) A successful attempt is the solutions Andreescu and Dospinescu elicited for their book "Problems from the Book": they created an Art of Problem Solving forum http://www.artofproblemsolving.com/Forum/viewforum.php?f=584 where people could post and discuss solutions, and then published a solutions manual (and more) - "Straight from the Book" including solutions from many of the contributors.
(b) http://mathbooknotes.wikia.com/wiki/Math_Book_Notes_Wiki (it would be great to develop this into a collaborative wiki for solutions). 
