Since Geany 0.12 there is a basic plugin system. You can get more features into Geany and developers can easily add new features and/or improve existing ones.
The Geany-Plugins project is a combined release for each matching Geany release, starting with version 0.17.
It includes a huge number of plugins for different purposes.
Please have a look at http://plugins.geany.org/ for more information.
If you wrote a plugin and it should be listed here, please email the project admins.
There is an plugin wishlist page, but be aware this may contain malicious links.
The Plugin API documentation is currently incomplete, but already contains useful information. Please read the HACKING file for instructions on how to generate it and any related information. The generated API documentation can also be viewed online at http://www.geany.org/manual/reference/. In case of any questions, feel free to ask us on the mailing list.