A very interesting too for sound effects generation totally free and open source

A program for creating sound effects for games, music, videos – whatever you want!
A manual and some example sounds are included to get you started.

Works on Windows (requires .NET Framework 2.0) and other platforms through Mono.

Features

• Multiple channels: Up to eight individual channels. All or any element of a channel can be copied to another.
• Modulation: For vibrato and tremolo (pulsating variation in pitch and volume).
• Envelopes: To shape everything the way you want it.
• Waveforms: Make your own custom waveforms, or use any of the standard ones (sine, triangle, sawtooth, square, noise).
• Effects: Add cool post-effects to your sound.
• Customizable randomizer: For the lazy sound designer.
• Customizable mutator: To randomize your sound slightly.
• Batch save: If you are in a hurry and need a lot of sounds fast.

http://labbed.net/software/labchirp/