A problem that I’ve had since upgrading to 10.8 Mountain Lion has been with the Open With menu. I find that over time there is a build up of duplicates. Primary offenders seem to be: Pixelmator and Evernote. This seems to solve the problem:

/System/Library/Frameworks/CoreServices.framework/Versions/A/Frameworks/LaunchServices.framework/Versions/A/Support/lsregister -kill -r -domain local -domain user
killall Finder
echo "Open With has been rebuilt, Finder will relaunch"

Then Relaunching the Finder with ⎇+cmd+esc should solve the problem. However I found that the issue kept coming back. I just decided to remove all the apps and assign apps manually. You can do the same by following this guide here.