In order to keep track of all of the paperwork, members and purchases regarding the co-op, we wrote our own in-house software. In the spirit of the co-op we released under an open source license and we'd love others to fork it and use it if they feel it can help them out.
It's a web application written in Grails, and runs on Tomcat with a MySQL back-end database. This software stack is also purely open-source. We use Ubuntu linux to host our management app.
Please send us an email if you're interested in using this software and we'll answer any questions you might have.