I keep meaning to. Most of those out there are crappy, but what I want to do is too big for me to code. (Think: A cross between the exokernel project, Plan 9, MOSIX and Linux, but mathematically proven correct and security-hole free using higher-order logic, with a GUI that delivers what Berlin and NeXTStep kept promising, some of the activities benefits of OLPC, the ability to set up pipes between GUI applications graphically and the ability to define fonts using the power of metafonts but with all the kerning, anti-aliasing and attributes of modern font engines and all the support for alpha blending and high dynamic range of modern graphics systems.)
Well, I could code it, but only if someone paid me to do nothing else for the rest of my life, and supplied me with a thousand year's worth of the elixier of perpetual youth.