When I wrote my plugins for tracking the visits to each page (hitcounter), presenting the counts by popularity (favorites), and displaying the most recently read posts (lastread) I used Perl's Storable module as a simple way to serialize the data structure to and from disk. At the time I wasn't too concerned with performance, my blog agilitynerd.com didn't suffer under the load of the disk reads/writes.
But periodically I get DOS'd and my website becomes unresponsive. Not too suprising since Blosxom has to run as a plain CGI script; but I'm certain the disk writes of these plugins weren't helping. Occasionally the Storable on disk would become corrupted which would take down the site (not enough exception handling in the plugins).
I decided to go with Mongo as the backing data store to remove the disk reads/write from the cgi script. It supports an increment operation (ala memcached) so multiple threads can update the hit count and MongoDB will "do the right thing". It also has a query API that made rewriting favorites and lastread trivial.
I've hosted the plugins on github: http://github.com/saschwarz/blosxom-mongodb-plugins The mongohitcounter plugin can be configured to automatically import existing count data from the hitcounter plugin's Storable. So conversion to the new plugins just requires updating the template's variables to the new plugin names.