**** Note, if you cannot find SFXR at the ludumdare blog any more, you can find it http://www.woolyss.free.fr/chipmusic.php#generators . This site is a great source for all kinds of free music and sound generators.
I stumbled upon this tonight and I can't say enough good things about it. The ludumdare blog has a new piece of incredible software that uses old school subtractive synthesis to allow ANYONE to create 8-bit game sounds effects EASILY. You can even save the sound out to a the file as an 8 or 16 bit wav file in 22k or 44 kbits/sec.
You can choose from any of the three standard wave form patterns (square, sine, and sawtooth) plus a random noise generator wave pattern. You can then modify the sound with 23 different slider bars that represent various filter, decay, delay, and frequency settings. On top of that, you get to choose from 7 preset sound types to get you going : Pickup/Coin, Laser/Shoot, Explosion, Power Up, Hit/Hurt, Jump, and Blip/Select. As if that wasn't enough, you can create completely random sounds, and also slowly mutate any sound. The mutate is awesomely effective at actually creating different pitch and frequency music loops that can be combined in Acid, Fruityloops, etc to make 8-bit music!
Here are 4 demo sounds I created and exported in about 60 seconds:
This works in Windows and they also have a native Linux port!