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Quite possibly the best free music notation software currently available.
Excellent music typesetting programme. This is much more compact than sibelius (and it's free!) and outputs some beautiful scores. The interface is similar to that of python and latex, so if you've had some experience with these, then the lilypond language will come naturally to you.It's recommended that you keep a copy of the learning manual handy (available from lilypond) because the language can get quite complex. You should also download an editor to help you with the process (i'm using frescobaldi).
Pros: beautiful printouts of scores
Cons: may take a while to understand how it all works