This dead simple drum machine that runs in your browser is so much fun.
The HTML5 Drum Machine borrows its aesthetics from classic beat boxes like the TR-808 Rhythm Composer. It's got five different sound banks: Hip hop, electro, house, techno, and acoustic. Each bank has 13 different sounds for which you can tweak the individual volume and tone. Pick your bank, hit play, and lay down your instruments on the 16-step sequencer interface that runs across the bottom. After you've laid down your beat, you can export it as a Wav.
Even if you don't have any experience with drum machines, this simplified beat maker is intuitive enough that you'll impress yourself within just a few minutes. You can probably figure it out on your own, but if you're a total noob here's a tip: The important thing to remember is that for each of the 16 "steps" in the sequence you can play each of the 13 sounds in a bank. So when you click on one sound—say the Kick— you can place that sound in any or all of the 16 slots. When you click on the next instrument, you again have the option of sticking it in all of the 16 slots.