FL Studio (formerly known as FruityLoops) is an advanced music synthesizer which includes an overwhelming amount of synths, effects and has VSTi support. The software is renowned for its ease of use compared to professional packages such as Cubase and Reason.
There's two versions of it, the Demo version which is free to download, and the Full version which you need to pay for it. On the Demo version once you save a project you cannot reopen it until you buy the Full version, though you can still export them as a MP3 file.
It is also one of the many programs that are commonly used to make YTPMV's.