Redmine

The installation and usage of plugins is currently limited to the system's administrator, however it would be an amazing feature (and would require a complete re-writing of how plugins get implemented) if the Redmine administrator could upload, install and configure a plugin via the control panel online on the web interface.

i.e. How popular CMS allow installation of modules etc.

This is something I personally would love to see.
I hope this post isn't already posted, I searched for it, my apologies if it is.