Over time, when lots of applications have been installed on a Mac OS X machine, the "Open With..." contextual submenu for files has a tendency to grow to quite an uncomfortable length, especially on older machines, which require a long time to generate and draw it.
One method of shortening the menu is to simply trash the applications you don't use. However, there are many vital applications that are set to be able to open common file types, such as .txt, .avi and .jpg. For example, how often do you open a JPEG file using QuickTime player or Project Builder? but you wouldn't want to trash those apps, either. The following is a way to avoid having these apps clutter your "Open With..." submenu without deleting the apps themselves.
Control-click (or right-click) the app in question and select "Show Package Contents"
Open the Contents folder in the app bundle. There you'll find a file called Info.plist. This is an XML property list that stores all sorts of information about the app.
Open the file with your favourite text editor. I recommend Hydra, but TextEdit will do just fine.
Search for something that looks like the following:
and so forth, with the suffices the app is able to open contained within the tags. Then you simply delete the suffix for items you don't want the app to open.
Save the file, and close it (or quit the editor).
This will prevent them from being listed in the "Open With..." menu. The application will still be able to open the relevant file types. However, you will no longer be able to drop files with the suffices in question onto the app directly.
Directly from Mac OS X Hints