The Cybermetrics Lab of the Consejo Superior de Investigaciones Cientóficas has released The Ranking Web of World Repositories (January 2010 edition).
Here's an excerpt:
As in previous edition we provide two global Rankings. One that covers all repositories (Top400) and another that focuses only on Institutional Repositories (Top 400 Institutional). We are considering to include in future editions portals of journals and papers (super repositories).
The composite index (World Ranking) is computed combining normalized values instead of ranks. The visibility is calculated giving extra importance to the external inlinks not coming from generic domains (.com, .org, .net). The figures for rich files (pdf, doc, ppt, ps, and new for this edition, xls) are combined and not treated individually.
Read more about it at "Ranking Web of World Repositories ."