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.
This IS where you install programs that are published (as opposed to "assigned") via GPOs.
I've been searching for the past 30 minutes to find the answer to your question and it appears that there is not a single shred of documentation on installing a program from the network other than this ridiculously non-detailed page: http://windows.microsoft.com/en-US/windows7/Install-a-program At first I believed it would be publishing apps via GPO's but this isn't the case.
It would seem as if there is a specific way to "advertise" applications. That's the term I keep finding. However, I cannot find any information beyond that. I'll keep looking.