I'd like to have functionality like this in Linux, Xorg, whereby I can sketch on the desktop as in the example image below. Anybody have suggestions?
It's available in the Debian / Ubuntu repos at least. See my related answer over at askubuntu.com StackExchange for installation and usage under Ubuntu. (In short: use Pause instead of F9 under Ubuntu.)
It seems such a tool exists for GNU/Linux already. It's called Ardesia. Read about it here
Example (taken from the above link):
Also, note that
ardesia seems to have made its way into the repos (it's in the Debian stable repos, at least).