More and more applications are moving to the cloud: Google Docs for productivity apps, Meebo for instant messaging, Gmail for e-mails, Salesforce for CRM, etc.
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Giving javascript control over right-clicks gives you something like this: http://periodic.lanl.gov/elements/24.html. I really love this website, but its attempts to keep me from copying text or images (whatever it's trying to do) seriously interfere with my web usage patterns. I always open things in other tabs. I always the right click menu to access the "back" command.
It also irritates me to no end when some web site has a flash animation somewhere that steals my control key (so I can no longer Ctrl+Tab to flip to another tab.
My bottom line: web applications can't supersede the local computer's built in commands. If a web application starts taking over control keys, right clicks, etc., it has crossed the line between local and remote applications. That is a very important line to keep crystal clear.