Looking for that elusive online score that might possibly be in the public domain? The Internet Music Score Library Project (thanks, Musical Perceptions) is a wiki that contains links to thousands of online scores in the public domain that can be quickly saved or printed. There's a ton of interesting stuff here, but I'll play the contrarian and make a case for the old-style published scores for the following reasons:
1. Sure, print out your free score. However, unless you print it out on acid-free paper, it won't last long because most printer-grade paper lasts only a few years before degrading. Also, if you accidentally recycle your printed out scores, you've lost not only the score, but your fingerings and other markings too.
2. Unless you're a paper-binding specialist, the binder you store your scores in won't be as reliable or attractive as a published score, and the three-hole-punched pages won't stay in as firmly as they do in a printed score.
3. Part of the process of becoming a performer or teacher involves building a large library of scores over time. This will be a resource you can draw from for the rest of your career. Well-bought scores such as Urtexts will last for the length of your career, have an easily found place on your shelf, and look professional on your shelf, much more than a stack of binders will.
4. Is the public domain score the best one for you? Remember, there are two parts to a score that involve copyright: the music itself and the publisher's plate on which it is printed. Often earlier scores aren't of the same quality as later ones that bear the fruit of much scholarship. Take the example of Beethoven piano sonatas--I would much rather use a proper Urtext edition than a much less accurate public domain edition (a late 19th century Breitkopf & Hartel?).