Zulupad and Zulupad Pro "are now completely free." You can download Zulupad from the Website, but the link for Zulupad Pro does not work. It's also on Github. I was never very impressed with the application, as you can see here, but you can't beat "free."
The entries are kept in xml files that can be edited in an editor, if desired.
I do hope that the demise of this software does not indicate a decline of interest in personal wikis, but I am afraid it does.