Symfony framework comes bundled with many great plugins, they can be extremely useful for your projects' functionalities. But there are very few plugins that provide documentation functionalities. While developing a big project, it's really easy to get messed up with working on dozens of model classes, modules, actions, etc.
There are few plugins I would like to recommend:
This plugin is an alternative to sfPhpDocPlugin. Doxygen seems to be the best cross-platform documentation tool available, including lots of configuration features (one of them is easy UTF-8 rendering, in comparison to phpDocumentor which still lacks support for any encoding different than ISO-8859-1).
You only need to have doxygen installed, execute few tasks, modify the configuration and your code documentation is ready. Remember that a developer has to go back to the application code even few years after finishing a project. If no documentation is found then, applying even small changes is... quite difficult.
The last two plugins are based on GraphViz, a wonderful graph rendering tool. The plugin is provided with only one task and is easier to use than sfDoxygenPlugin. After installing you only need to run one task and the application map is already generated.
What is this application map? It's a graph in which each element of the controller has it's own node. That is, the project-root node has child apllication nodes (e. g. backend, frontend), application nodes have child module nodes (including both admin generators and custom modules) and, finally, module nodes have child action nodes. Each node is labeled with the name of the project, application, module or action, depending on the node type. Each action is additionally provided with the documentation code. All this makes sfApplicationMapPlugin a very useful tool when you want to take a brief look at you application and you don't like browsing lots of files - the plugin does it for you and generates nice images. Take a look at the plugin readme to see example application maps(#1, #2).
Finally, database schema has to be documented as well. Many developers tend to use good old FabForce's DBDesigner 4. But, as it usually happens, many tiny differences are made during development, time is money and updating your DBDesigner schema is definitely NOT money, therefore the schema image is not up-to-date...
Why using DBDesigner and spending your time on updating a file which won't be used in your project, when all the database schema information you need is present in schema.yml files? This is where the sfDoctrineGraphvizPlugin comes to help. Just like sfApplicationMapPlugin - just install, run one task and the images are generated for you.
During site development, a developer usually doesn't care about documenting the project. But if he leaves it for a long time, or when a new developer comes to replace him, understanding of a project becomes a big problem. Don't forget to provide a good value documentation for you projects, it takes very little time - and eventually saves a lot of it.