Windows/Linux only: LMMS puts a powerful set of music-making tools into one window, letting those who can’t swing a home studio roll samples, bass grooves, synths, and other effects into high-quality audio files.
The obvious comparison here is to FL Studio, the synth and sequencing software fondly remembered by many a college tinkerer as FruityLoops. LMMS supports file imports from both FruityLoops and its suit-wearing brethren, along with many other audio standards, and should work with any MIDI tools you’ve got handy.
If none of that made much sense, fear not: LMMS is ready for you to start messing around and gluing together your sound samples, melodies, and other audio toys in your quest to be the new Dance Party King (or Lord of the Headphones, depending on your musical ideology). The LMMS Wiki has some handy tips on getting started and figuring out what all those little buttons do.
LMMS is a free download for Windows and Linux systems (pre-rolled for many distributions at the download page). Got another free tool you use for musical tinkering? By all means, share the link in the comments.