The new Ableton Learning Synths website is a great resource for getting your sound design fundamentals together.
Learn basic concepts like amplitude, pitch, envelopes, modulation, oscillators, LFOs and filters. You can even create your own sounds. And if you're already a synth pro, the Playground section lets you dive in and experiment. There are even preset sequences and sounds. And if you're a Chrome user, you can connect your MIDI keyboard.
For more information, visit the Ableton Learning Synths website.