VGA Simulator
Use the tool online now, http://ericeastwood.com/lab/vga-simulator/
See this blog post for more info and how to use: http://ericeastwood.com/blog/8/vga-simulator-getting-started
The code is a bit crusty and old now but happy to have it on GitHub. It could also use a lot of optimization to make it faster, feel free to submit a PR.
Python port (CLI)
Pedro, @pvieito made a CLI version of this app in Python that you can find in this gist.