Didn't know about WikidPad. I downloaded it and will be playing with it. Of course, MediaWiki and XAMPP (or a server) is an interesting possibility for other reasons.
If you want your wiki to be public, or accessible from multiple computers, MediaWiki (along with dozens of other web-based wikis) is a good alternative, sure.
I like WikidPad because it's _fast_ in Edit mode (on-the-fly text formatting, like code highlighting), and it turns out to be a killer feature for me: without this speed I probably wouldn't be using it as much. With almost all other wikis I know, you need to switch back and forth between edit mode and preview mode, or they have a WYSIWYG editor mode (and I avoid that as much as possible -- just a programmer's obsession... I like to deal with the source).
I haven't used TiddlyWiki much, but I loved the concept when I first heard of it (self-modifying single file). It sure is a good way to get started fast.
I also came across this: http://tiddlywiki.com/