As my first attempt at hacking WordPress (or coding in PHP, for that matter) I've written a hack that extends the post formatting rules to allow you to create wiki-style links between posts.
In a nutshell, you simply put the title of a post inside double square brackets,
[[like this]], and it becomes a link to that post. There are some optional bits of syntax to restrict the lookup to posts in particular categories, to make the text of the link different from the post title, and to add a tooltip to the link.
Of course, this isn't nearly all of what a real wiki does. But promiscuous interlinking is cool -- you can use it to make a kind of glossary in your blog and easily link to it from other posts. I think I'll be working on other wiki features in the future.