SDMove finds apps on Froyo that can be moved to external storage (SD card) and provides an easy way to move them.
Not all apps are shown, only apps that have a listed preference for storage location. If no preference was made by the application, it cannot be moved.
For long-term maintenance, install SDWatch, which will immediately notify you if a newly installed application can be moved.
Updated to support Gingerbread. There's no need to update if you aren't running Android 2.3 or later. (Other than to get rid of the update notification.)