Music Maker Jam is a Windows 8 Metro app that lets you quickly create your own songs live, without any musical knowledge.
This isn't designed with experts in mind. Music Maker Jam lets you create tracks in three genres: dubstep, tech house and jazz. Each style opens an 8 channel mixer, with each channel featuring a variety of different sounds or instruments. There's a sequencer for controlling the harmonic progression of your song too. You can add effects on the fly as your song is playing.
Music Maker Jam will have you making songs in moments, but that immediacy brings with it some serious limitations. It is a lot of fun, but the small number of sounds and samples means you will quickly run out of inspiration. However, Music Maker Jam is a lot of fun, and is a great way to get into making music.
Music Maker Jam is even better if you're using a Windows 8 tablet as multitouch is supported, which gives you much greater control over the music.
Great fun for just messing around, and a good introduction to making music on computers, Music Maker Jam is a definite download for Windows.