I making something sort of like this for Mac. It's for recording a selection of your screen with the system audio. I have not made it so it can convert to Gif or WebM, but that is in the plans!

https://github.com/Jonovono/cutter.

LICEcap [1] does this beautifully.

> LICEcap can capture an area of your desktop and save it directly to .GIF (for viewing in web browsers, etc) or .LCF.

[1] http://www.cockos.com/licecap/