Almost every project has a website. There is often valuble information at the website concerning the latest release, new features and upcoming changes. Most sites also have a documentation section that includes installation documentation and some application documentation. As in the documentation in The Linux Documentation Project, mileage will vary. The real gems in the project web sites are found in the forums and the mailing list archives.
A growing number of projects have wikis on their sites. A wiki is web page or set of pages that can be modified by just about anyone. Some projects have started using wikis for their documentation allowing the users of the software to update and add to the documentation. There are also variations on the normal wiki where users add comments to the documentation pages. The user added comments often provide valuable insights and solutions to specific problems.
Most project sites include forums where users can post questions and comments on the software. Most of the time, someone else has already had whatever problem you are trying to solve, and you can often find a discussion about it in the forums. If not, you can always pose your question here and often receive a speedy response.
An older and some would say surer place to find the help you need is in the archives of the mailing list. Each project usually has one or more mailing lists. As with the IRC channels, the names for the mailing lists are pretty much standardized. There is usually a general discussion and announcement mailing list, a development mailing list, and a user mailing list. On smaller projects, a single mailing list might be used for everything. You can subscribe to the mailing list and post to it by sending an email to the mailing list's submission address. As in forums, this is a great way to get help when you can't find your answer any other way.
The mailing list archives are often the true goldmine of information about solving problems, though. In the longer lived projects, these mailing lists archives go back years and just about any problem that anyone has ever had is discussed at some point. Since the mailing list archives are usually searchable, it's easy to find the information you need. Often when a Google search turns up helpful results, it's because it found what you needed in a mailing list archive. You often have to subscribe to a mailing list in order to search and read the archive. It's well worth the couple of minutes it takes to sign up. You can always unsubscribe once your problem is taken care of.