Ask HN: best low-latency desktop sharing application

I am looking for a desktop sharing application. Sometimes I just need to show the other guy what I am looking at and vice versa. I need something that is as low latency as possible. Free is preferred but will pay if the product is worth it.

The sharing is between windows and mac osx.

An acceptable solution would be something that limits shared space to a portion of the window for efficiency.

The #1 goal is low latency.