In Windows there is a place under the Control Panel called Get Programs and Install a program from the network (in Win 7 at least), possibly a slightly different name in Vista, but it was there too.
The question is basically: How do you publish programs to that so users can go there and install?
Bonus follow up: Is it a good way to deploy optional programs? (Compared to using GPO's.)
In Windows 7 you can go to Control Panel, Programs, Programs and Features, and on the left hand menu: Install a program from the network.