Open Source cross platform music composer/score wrighter.
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Contrary to most of its commercial counterparts, MuseScore is lightweight (less than 100MB) and can run acceptably on aging computers.
Pro MusicXML import and export ensures a degree of compatibility with other programs
If you are submitting files to a publisher, MusicXML format can be read by the big engraving programs: Sibelius, Finale, MuseScore, etc. Often there are some display issues that will need to be tweaked when using MusicXML between different programs, but publishers will spend a lot of time tweaking the file anyhow.
Pro Easy to add new sounds
If you're not happy with the default sounds (a good, but aging set from Roland), you can import any .sf2 and .sf3 soundfonts and easily use them. You can even have multiple soundfonts running simultaneously.
Pro Free and open source
The pricing means you can dump it on any computer you like, without having to empty your wallet for a license. It also means you have a chance to use this program as a starting point for your own engraving software program, if you are also a programmer.
Pro Great sheet sharing web page
So you composed a new score! Now what? Well, you can start by putting it on the muse score sheet music sharing web page where others will be able to enjoy it and comment on it. Or maybe you're just looking for a score from an old video game: in that case, you should probably search the page for it, and download one of the many available formats (pdf, muse score format, mp3, xml, etc).
Con No transcription tool
If you are looking to play your MIDI keyboard and have the computer attempt to transcribe and print out your latest masterwork as you play, MuseScore is not the tool for you.
Con Bad sounds
Although its ok for basic composing, the default sounds can sound inferior to other more polished sounds. In fact, it will remind you of old mario games with midi music; because that is pretty much what it is.