SFXR is a free simple tool for generating lo-fi sound effects.

This one’s great for generating a bunch of lo-fi sound effects really quickly for placeholders or whatever. You can also run these through other audio filters/pedals and get some interesting stuff. SFXR supports square, sawtooth, sine, and noise waves and lets you export the sounds to wav. You can also save your slider settings if you want to update the sound later. It has presets for generating common game sfx like SHOOT, POWERUP, and EXPLODE. One feature I wish it had is a waveform display and some way to interact directly with the display. For this reason SFXR is more of a first step in creating a sound than an all-in-one solution.

Tip: Select a type of sound you want (like JUMP) and hit the button over and over till you find something you like. Then select what type of waveform (SQUAREWAVE, SAWTOOTH, SINEWAVE, or NOISE) then adjust the sliders to perfect it, or just hit MUTATE until it’s closer to what you want. You can also hit RANDOMIZE to get a totally new sound and then mutate off that, or just draw out your sound from scratch with the sliders.