Splice Music, a Flash-based online sequencing tool, opened to the broad Internet public about a year ago. It's free, although registration is required, and filled with thousands of sound samples and remixable songs, all licensed under various Creative Commons licenses, meaning that they're available for you to recombine. It's a fun place to start for old-school analog musicians who've never played with a sequencing program before. (And who don't have a Mac with Garage Band preinstalled.)
Remixing other users' songs is one way to learn how the tool works, but if you're like me and prefer to start with a blank slate, begin by navigating through the sounds library, and click the "plus" button when you find a sound you like. I'd recommend sticking with fairly short sounds--even just a second or two--rather than long sequences, as short sounds are easier to blend together into a coherent track. The search function isn't perfect, so you'll probably want to navigate through the library manually until you've got a toolcase with a couple dozen sounds.
Then, get to the core of the site: the sequencer itself. (It works best with the Firefox browser, and requires Flash 9.) Open one tab with the FAQ and another with the sequencer itself--that way, if you get lost in the sequencer, you can always refer back to the instructions without navigating away and losing your work.
In the sequencer, begin with the "add stuff" button, select "instruments" from the "what do you want to add?" dialog box, and drag the "audio track" bar all the way to the left. That creates a blank bed for your first audio track. Then you can open your soundcase and drag and drop sounds onto that audio track. You can layer multiple audio tracks atop each other, each with its own type of sound, edit the sounds themselves, change the tempo of the entire piece, and so on--it's really a playground for audio experimentation. You can also create melodies with the piano and sinusoidal synth (other sounds are coming soon, according to the FAQ).
Once you're familiar with the tool, you can try recording your own sounds with your PC's mic, or uploading sounds you've previously stored on your PC. Finally, when you've got a track you're happy with, you can save it and do things like embed it into a MySpace page.
Even if you're not interested in creating a song, this is an interesting tool from a business standpoint as well: Splice shows the promise (and limitations) of rich Internet applications. No, you'd never use Splice for serious commercial audio production, but it's a great way to learn the ropes and create some reasonably sophisticated tracks. This is why Microsoft is spending billions embracing online services and creating its own Flash competitor--it's impossible to ignore how sophisticated Internet-based applications have become.