Postcomment
Angela Beesley's wiki
After getting too much spam with ArticleComments, I tried Travis Derouin's Postcomment which works with the SpamBlacklist. After installing it, I got far more spam on talk pages, presumably as a result of having an open edit box on the page. When I installed ConfirmEdit to deal with this, I found the captchas were not shown if spam was added via the comment box.
My adaptions to the extension whilst I was using it are here. I removed the usernames since people can't log in here and added a subject line which gets turned into a wiki header. I also adapted it to make the signature formatted like a wiki signature normally appears rather than the forum-style names above the post.
You can get the extension from mediawiki.org