Framework Needed: Review System?

I'm in the market for a robust review system for several commercial and non-profit sites that need to be developed.
Ideally, the solution would be something as follows:

• Open-source
• PHP/MySQL/PostgreSQL based
• Modular architecture for custom development

Open-source would be most appreciated for my non-profit clients that can't afford cost-prohibitive licensing. Another benefit would be that hopefully the code would be flexible for implementing custom features. If it were robust enough, it would surely do for commercial deployment as well.

That in a nutshell would be the ideal solution.

I'm fairly new to the open-source world. Is there some massive online forum that would be a better place to put in a request like this? There are several projects out there that have review based components, but they are all plug-ins to other CMS's, instead of a stand-alone project.

However, if such a project(s) doesn't exist in the open-source world/web dev world, licensing would still be an option. Where would one find a matrix of various systems to purchase? Numerous Google searches produced irrelevant results. It seems that everything under the sun has been "reviewed", except the review systems out there.

The one that seems the most slick (so far of the commercial breed) is the following:

http://www.randommouse.com/cgi-bin/...out_product.cgi

Surely on this forum there has to be some medium to large developers that have perahps started out with a more structured solution and then custom developed your own? What would be a good compromise of breaking in a new system? I would really prefer not to invest to build one from scratch. Please feel free to post your own review-type sites just for practical example. Thanks again for the pointers!