LMMS - Make music with a free, cross-platform tool
SoundGrail - SoundGrail Music App designed for pianists, guitarists, and DJs.
GarageBand - GarageBand is a fully equipped music creation studio for Mac
keezy - A colorful soundboard. Play with music.
Ableton Live - Ableton makes Push and Live, hardware and software for music production, creation and performance. Ableton´s products are made to inspire creative music-making.
Figure - Propellerhead creates world-class software products and services that inspire music makers and provide the foundation for a worldwide creative musical community.