One of the best examples I've seen so far is the ARSList, a very active mailing list focussed on Remedy's Action Request System. ARS is a amazingly capable and customizable application solution (Help Desk, Asset Mgmt..., etc.) and there are as many different ways it can be used as there are businesses today.
The ARSList has helped to foster and archive an extremely active and helpful developer base. Remedy itself has helped this process by maintaining an open source API, a knowledge base of known issues and ideas, and by showing the wisdom and vision to incorporate field-developed technology into their releases.
Admins and consultants can share their experiences and bounce new ideas off of each other, which increases their value and abilities. Customers are serviced by more capable and efficient admins and consultants, which reduces their project times and costs. Remedy itself reaps the benefits of new technologies created and tested in the field, making for a very robust and customer-tuned product.
In addition to the demonstrated advantages of peer networking association, I think there's also a corporate lesson here: Companies that support the creativity and needs of their customer base are healthier and more robust. (There have been times in the past few years when it's seemed that we've kept Remedy alive by sheer force of will.)
My advice then, is to always try to create and maintain strong ties between your association and the product(s) it supports. The benefits to both can be enormous.