The code is built on the codebase from the weekly timesheet plugin so we want to send many thanks to its author!
We are happy to give it back to the public so anyone can download and use.
Our version is based on the wk-time 0.0.8 and we use it with 1.4.2.stable.9693 and you can install itjust as any other redmine plugin. After the upgrade to Redmine 2, we'll post here also the updated version.
Update (June 2013): We stopped using weekly resp. monthly timesheet as the "Spent time" functionality of newest redmine is sufficient.