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

LabChirp

Sound effect generator

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.