sfxr

sfxr is a tidy little sound effect generator, made by Tomas Pettersson (AKA: DrPetter).

Ports

Versions of SFXR ported to different platforms, and/or with additional features added.

as3sfxr is a Flash port by Tom Vian as. They've also made a similar tool called ChipTone, which has browser, Mac, and Windows versions.
Stephen Lavelle built-upon as3sfxr as as3sfxr-b. He's since enhanced it further as Bfxr, which in addition to Flash has Mac and Windows releases. It looks like there's a JavaScript version in development at github.com/increpare/bfxr2
sfxr-dssi is a DSSI version by Sean Bolton. DSSI is a plug-in format you can use in a DAW.
usfxr is a C#/Unity port by Zeh Fernando, for generating sound effects in real-time in games. It's no longer being developed, so I'm unsure if it still works.
Jfxr is a browser-based port. Give syu more options to play with, and a nice addition is that it remembers sounds you've made (but not mutations), so you can go back to them.
jsfxr is another browser-based port, it's source code is on Github.

Not quite ports, but work in a similar way.

sfxia - Similar idea, but simpler.
SoundDevX - More complex, works in browser, exports .WEBM.
LabChirp - More technical, for people who understand audio on that level, or want to learn.
ChipTone - Much more technical, but you still have categories you can use as starting points, so you don't actually need to understand any of the fancy stuff.