Download Hammerhead Rhythm Station (see Resources). This application specializes in drum-based beats, though you can also import sounds you've recorded or downloaded and insert them into the rhythms you create.
Install FL Studio (see Resources). The FL Studio suite not only allows you to create drum-backed beats for your next song, it also lets you insert different types of instruments. You can set these instruments to play certain notes and, depending on your musical expertise, you could very well put together an entire musical track using FL Studio.
Visit the JamStudio website (see Resources). JamStudio is an online, Flash-based rhythm creation tool for those looking to create beats for a music track. When you're finished perfecting your beat, you can download the file to your desktop.