Reflow bills itself as "music notation for the rest of us" and I have to say the app packs a punch for musicians and students of music. Reflow's notation software has a lot of great drag-and-drop functionality for quickly building compositions, and takes away some drudgery from the process. Plus, it features iCloud sync so you can quickly take your Mac work on the go via iPad or iPhone/iPod touch.
Here's creator Sébastien Bourgeois telling me how Reflow came to be in this week's Origin Stories (recorded in October, 2012 at MacTech conference).