If you're familiar with Maven, you may be wondering why anyone would bother with Forrest. Maven does everything Forrest does, and much more: completely automating your build, attaching JUnit and CVS reports and all sorts of additional useful information on top of the basic documentation. It even provides targets to auto-install your newly generated web site onto a remote host.Read Documenting Projects with Apache Forrest
I have used Maven on a number of projects, and it's an impressive package. The learning curve is not much worse than Forrest's for basic use, and since you don't have to learn Ant if you use Maven, it's arguably even less for setting up a project from scratch. Forrest as a whole is less complex, though, and if you don't need everything Maven provides, you might want to start with Forrest and migrate to Maven later if you need it. Forrest is also better if you have an existing large-build system based on Ant: it lets you add in Maven-style web site generation incrementally instead of rewriting all your build scripts to Maven-ize the project.
In terms of pure-documentation alternatives, another solid option with a lot of open source community support is DocBook. You could write the manual for a 747 with DocBook: it's the ultimate SGML (or XML; there are two versions) dialect for technical writing. The XML variant has a nice set of stylesheets from Norman Walsh that can generate HTML, PDF, RTF (Microsoft Word) and other formats from DocBook source. I think Forrest's XML dialect covers 80 percent of the cases, with a much smaller learning curve, but for a large project that also needs to produce print documentation, DocBook merits consideration. Note that if you want to migrate from DocBook, Forrest supports rendering a subset of DocBook/XML as well, but it is not well supported. Forrest does not aim to become a full-fledged DocBook renderer any time in the future, either, according to one of the developers, so I would not rely upon it as a format for new documentation in Forrest.