If you've made good use of your Mac's "Open With" contextual menu, you've probably noticed it can get a bit cluttered with duplicates. Fortunately, there's an easy trick to tidy things up.
When this happens, it usually means your Mac's LaunchServices database needs to be rebuilt. You can do this quickly by launching the Terminal and entering the following command:
/System/Library/Frameworks/CoreServices.framework/Frameworks/LaunchServices.framework/Support/lsregister -kill -r -domain local -domain system -domain user
This works with OS X 10.5 and higher, but if you're on an earlier version of Mac OS X you'll want to try the following instead:
/System/Library/Frameworks/ApplicationServices.framework/Frameworks/LaunchServices.framework/Support/lsregister -kill -r -domain local -domain system -domain user
The Terminal will spit back a bunch of stuff after telling it to rebuild your LaunchServices database, but you can just ignore it. The important thing is to keep the Terminal window open until it's finished doing its thing. When it's done, your "Open With" menu should look a bit slimmer.