This website provides data on the number of records in UK Institutional Repositories over time. The data was collected from late summer 2006, and has been collected weekly ever since.
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 selecting 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.
The open access movement:
Putting peer-reviewed scientific and scholarly literature
on the internet. Making it available free of charge and
free of most copyright and licensing restrictions.
Removing the barriers to serious research.
I recommend the OA tracking project (OATP) as the best way to stay on top of new OA developments. You can read the OATP feed on a blog-like web page or subscribe to it by RSS, email, or Twitter. You can also help build the feed by tagging new developments you encounter.