Just a quick update to tell you about MuseScore. It is a free, cross-platform music notation software package that I have been trying this last few weeks and I have been really impressed by the way it works so far.
The most attractive thing about it is that it is free and the fact that it is open-source means that it should hopefully get updated fairly regularly as bugs get ironed out.
My experience so far is that it provides a sound alternative to the expensive equivalents like Sibelius. There are obviously a few bugs that are annoying but given the seemingly early stages of development they are easy to overlook given its overall capability.
The ability to save to its native format, as well as XML, and PDF means some impressive-looking scores can be obtained fairly easily.
To anyone who writes music, it is certainly worth a look if you don't want to shell out for the more well-known options.