redmine
Wrote one very small Redmine plugin (to copy some data to the clipboard in a particular format for export) and have just started another, much bigger one, to manage jobs (in terms of blocks of time sold to clients for budget management).
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Writing the Jobs plugin for Redmine is the first time I've got to use some Ruby features in anger-the ability to reopen classes and throw a bunch more code inside is incredibly powerful, and means Redmine's plugin system doesn't need to do a huge amount to let you do... pretty much anything.