How to share programs between users on same machine?

Always install all applications in the Applications folder.

If it's the kind you just double-click, then drag them all always (copy them) onto Applications folder.

The difficult question isn't sharing programs (which almost always are shared by default) but how to share data. In particular, music in iTunes, photos in iPhoto, videos in iMovie, they all resist sharing because those apps were not designed for it. In general, to share data you can use the "Shared" folder. Go to your home folder, and go up a level to Users, then select Shared. You might want to drag it to the places list on the left in the finder so you can get to it easily. You can put your iTunes and iPhoto libraries in Shared. To select the shared versions, hold down the Option key while launching iTunes or iPhoto. Video files can also be stored in Shared, however iMovie seems to be hard connected to an individuals Movie folder.