One of the interesting findings of my brief investigation of open access repository software by country was the heavy use of Wildfire in the Netherlands.
Wildfire was created by Henk Druiven, University of Groningen, and it is used by over 70 repositories. It runs on a PHP, MySQL, and Apache platform.
Here is a brief description from In Between.
Wildfire is the software our library uses for our OAI compatible repositories. It is a flexible system for setting up a large number of repositories that at the same time allows them to be aggregated in groups. A group acts like yet another repository with its own harvest address and user interface.
There are several descriptive documents about Wildfire, but most are not in English.