It is possible to launch an external application right from the Maya shelf. That could be useful when using an external tool like Photoshop, Mudbox or even Screen marker that I've talked about in here.
The way to do it is quite simple. Right in Maya, create a shelf button with the following code:
system("start C:/folder_location/sofware.exe" );
folder_location will be replaced by the complete folder adress where the software is located
sotware.exe will be the name of the software you're targeting.