Move programs and games to external storage
There are two ways developers can make their apps for Mac. The recommended way is to put everything the app needs into a "bundle". Apps created in this way enclose everything they need in the .app. As a result, these apps can be moved to anywhere you want, and they'll still work.


If you installed an app by dragging it's .app file from the DMG into Applications, or whatever folder it's in, it can be moved without any problems. Xcode is one of these kind of apps. Just drag the app anywhere you want. (Note: you may need to reset paths to your custom files once you open the app, such as pointing to the locations of your custom sounds, movies, whatever.)


The other way of developing apps involves using files external to the .app bundle. These apps generally have files in the system or user's Library folders or other folders, usually hidden to the average user. As a result, these apps must be "moved" differently - if they can be moved at all.


If you installed an app by double-clicking the .app or a .pkg file from within a DMG, it is very likely one of these more complex apps.  Creative Suite is an example of one of these. You might be able to move these apps in one of these ways:


use the app's installation process to select the new location, or
search around or contact the vendor to get the locations of all the external items you'll have to move, point to, or change in order to use the app from an external drive



Note: these are rules of thumb, so apply generally. Apps you get from the App Store are required to be in the bundle format, but that doesn't mean that once you open the app it doesn't store some stuff in other locations. I believe GarageBand does store information in some of the Library locations, for example.