OpenAhjo is an API and a UI for accessing the decision-making material of the City of Helsinki. The code is hosted on GitHub, and there are two versions of API documentation available: there is a human-generated, more high-level documentation, and an autogenerated API specification.
You can also have a look at the user interface, or browse the UI source code (written in CoffeeScript) to see how the API is used. The task list is hosted in Trello.
If you would like to report a bug, ask for a new feature or just have a question, you can either file an issue on GitHub or contact the main developer directly.
Background information on decision making process in Helsinki