I know a website which may be useful to you:
It's called ChipTone, you'll need Flash to run it, but you can generate sounds by picking presets and modifying other sound properties. You can then export the sounds you make as .wav files.
It's worth noting that this website is best used to make 8-bit sounds. I'm not sure what type of sounds you're wanting to make, but if you do want 8-bit sounds, then ChipTone is a good choice.
Also, like IGnormie said, Audacity is a good program to use, if you want to add further modifications.
Website:
ChipTone