Are there any open source resource monitoring tools similar to Window's Resource Monitor?
AlternativeTo.net doesn't list anything; or maybe I do not know a tool to search for to get started.
For Windows and Linux.
I'm not entirely sure what you're after, but provided you want the same information offered by the Windows' Resource Monitor, but on Linux, then consider the 'top' and 'atop' applications. There's also 'free -m' to look at the available, used and cached RAM; 'iostat' ('iostat 2 10' - ten runs every two seconds) for disk IO, etc.
If you want a graphical tool, then we need to know more about the Linux side of things, such as the distribution you're using and the window manager, etc.
For big fat solution, you could check http://www.cacti.net/. Beside tools mentioned by Michael, there are also