Munin API
Feature changes
split APC caching:
drush-based plugin: plugin is currently web-based, but this is only really necessary for drupal APC
generate the “ln -s” series of commands needed to activate plugins
New plugin ideas
Other extension ideas
Mobile monitoring (Android / iPhone). Munin templates don't cut it on a phone.
Actual access control for probes (currently just placeholders), in addition to the UI access control
Documentation
There is actually a book on the topic of Munin plugin development:

Third-party ideas
Existing products / services