Everything short of the customer data can be opened up, and even then the customer data can often be partially scrubbed and then shared for the benefit of all your customers.

One example was a Point of Sale system I worked on, we were able to simplify adding new products by creating a community database of UPC/other barcode formats that would have the name, description and other attributes of an item along with an average price. Very useful way to creatively transform this data, and help our clients do less work while accomplishing more.

With regard to opening proprietary code, we did a few credit/debit/EBT integrations, and one of the best choices made was to release our code and do development in the open.

When working with each vendor we could highlight exactly where our code was running into issues with their buggy hardware, and we were able to create straightforward non-NDAed documentation for these devices, which helped other projects that never had credit/debit integration. Additionally, our competitors didn't even end up fixing the bugs we highlighted in our documentation, I really thought they'd at least try to be a bit more competitive/feature complete.