there are oodles of sights that also have free sheet music, you can easily find these using Google
. There are classical scores that have passed into public domain, many modern composers realize they won't get known unless they give a little, and there's no better way to get your arrangements known than to take old melodies and whip up some classy arrangements.
It's alright to say you can just go buy these things, but this is a big planet, there are all sorts of problems. You will find many Russian sites where teachers have arranged modern tunes for school-level bands and posted them online because there just isn't any other way for those kids to get their hands on decent arrangements. Same goes for many places in South America where the school band traditions are very strong, but there are no international aid agencies willing to buy enough copies to go around. So they route around the problem.
A new phenomenon that I am starting to see here and there and which I find very encouraging is the electronic-delivered arrangements for very reduced cost, even as low as zero. Musicians with the skills and time have created printer-friendly copy that is intended for share so as to spread the joy; my 11 year old trumpet player here has discovered all sorts of great jazz material by completely obscure composers who have listed their works on the free sheet music sites. Seems to me free-scores.com is his favourite.