A wiki engine is software that lets you run a wiki. You can run it privately on your own computer to just take some personal notes, or you can install it on some server on the web for everyone to use.
Hatta is interesting in that it can do both things at the same time. You can have it installed locally on your computer (and preferably use Hatta Icon to conveniently launch it), at the same time you can have it installed on your server, and you can synchronize both installations. Moreover, your friends can also have local installs, and you can synchronize with them too! You can set your server installation in such a way, that anyone can clone it and have a complete copy (including page history) on their computers. Of course, you can also just keep it all to yourself.
Hatta is supposed to be a simple, little program requiring no configuration. I use it mainly for note-taking and project documentation. It's not suited for running encyclopedia or a content management system for your company, though. It's not even suitable for running a blog. There is plenty better software for that.
Hatta is the name of Mad Hatter from Lewis Carroll's books. It's also the name of this little wiki engine.
In the days of yore hatters often became mad because of mercury poisoning. This wiki engine is mad as a hatter because it uses a Mercurial repository to store its pages.
This site is powered by Hatta wiki engine itself, running the Development version, and is taken care of by Radomir Dopieralski. Most pages are editable, some are locked because of bots trying to edit them.