Hatta is a wiki engine – software that lets you run a wiki. It requires no configuration and can be easily started in any Mercurial repository.
Hatta's pages are just plain text files (and also images, binaries, etc.) in some directory in your repository. For example, you can put it in your project's "docs" directory to keep documentation. The files can be edited both from the wiki or with a text editor – in either case the changes committed to the repository will appear in the recent changes and in page's history.
This opens a whole array of possibilities. You can clone, merge and synchronize your wikis by cloning, merging or synchronizing the repositories. You can keep a local copy of it, and run Hatta locally on your laptop – then upload the changes when they are ready.