One of the most important tools for tracking the usage of files uploaded to Wikimedia Commons is BaGLAMa, which provides a summary of pageviews of Wikimedia pages that contain images from specific categories on Commons (see January 2012 report). It also provides for an easy way to compare the stats for any two months on record (see February 2012 report).
The tool has had trouble finishing its job in recent months, due to the stressed Toolserver. Magnus Manske, the developer behind BaGLAMa, has taken this as an occasion to review and rework his code, with the result that the tool can now be used again. He also re-calculated the stats for those earlier months where a record was missing, which resulted in an overestimation of the respective pageviews, since category membership was taken as of the time of running, not as of the month in question (which is difficult to implement and would drain even more sources from the Toolserver). The stats for December 2012 had been computed within a few days, and on the basis of the results for Category:Open access (publishing), it can be seen that the combined pageviews for images from that category rose from about 16.5 million last December (discounting an image featured on the Main Page of the Russian Wikipedia) to 23 million in December 2012.
In the following, I would like to highlight another use case of BaGLAMa - that of helping to identify articles that use a relatively large number of images from the target category. As an example, articles are listed that use at least five files from Category:Open access (publishing).