Is what i do is play the drums and the piano/keyboard and then add all the tweaks and effects in with acid music studio from sony. It was only £40.00 and is great software to use for the price. The nearest thing to that, but i don't think its even near the standard of music you can make with acid, is audacity.
Here is the link for Audacity,
http://audacity.sourceforge.net/
But, if your more of a sound effects person, you could use, like they said, LMMS. There is also some great software from SONY and Microsoft if you look hard enough. ^_^