The wiki has a heap of great articles for Java and Web Apps. If you want to contribute you can either create a new article or update an older article - testing on recent platforms and adding this information to the metadata or adding a buildable code example. Before creating a new article you should first search on the wiki to ensure you're not duplicating content.
The wiki is based on the same platform as Wikipedia and is hence very easy to use. If you do get stuck you can check out our Help and our wiki checklist.
So what to write about? I find that often questions answered on this discussion board are good for standard "How Tos" - like this one: How to detect if an app is running in Kid’s Corner
Other ideas are to think about core use cases you've run into problems with - accessing specific web services, information about porting to other platforms, great service libraries you've found and how to use them.
Lastly, as I said above there are a lot of older articles that are still relevant on todays devices - verifying that they work and adding ArticleMetaData and categories showing that they still work has a lot of value. THIS PAGE has articles which have known issues, or which may have issues - a good place to start.
Please add other articles you'd like to see, or write here.