Acid Express is a free 10 track only version of Acid 7.
For anyone who doesn't know Acid (originally produced by Sonic Foundry and now owned by Sony) is a leading loop based music sequencer/compositional tool.
If you want to give it a try go to: AcidPlanet
You will need to register to get the download but then it is free.
You can buy loops from Acid Planet but there are lots of third party loop libraries - and you can find plenty of free loops if your search the web.
Acid Planet also produce what the call their 8-packs. These consist of packs free downloadable loop packs which get updated with new loops (IIRC) every month. [Correction: weekly packs]
You can also download demo projects.
Why not give it a go and let us hear your masterpieces