(Thanks to Peter Suber and Charles Bailey for drawing attention to this item.)
This website provides data on the number of records in UK Institutional Respositories over time. The data was collected from late summer 2006, and has been collected weekly ever since. Since August 2008 is has collected data for Institutional Repositories worldwide.
The data is from the excellent ROAR based at the University of Southampton (ECS).
Where to start? Have a look at the table below (first link), it shows the number of records in each repository (registered in ROAR) for each week since July 2006. You can reorder the table, download the data (e.g. in to excel) and select individual repositories. Also check out the comparison page, which can be reached by first selectinig an IR on the right and then selecting an IR to compare with. Finally the info page is worth a read for details of what you are actually looking at, and issues with the data and presentation.