I've written several MediaWiki extensions, mainly for the CLUG Wiki. I don't think any of them are particularly beautiful or maintained, but they are an essential part of our wiki, and might be useful to somebody out there...
Written for ClugPark, this extension displays all the images in a category together, as a gallery.
<category-gallery category="PicturesOfPenguins" resizewidth="50" width="60" height="60"><category-gallery>
Mailman subscribe links:
Written for the Mailing Lists page, this extension displays mailman subscribe forms.
Obfuscate e-mail addresses:
href when you mouse over an e-mail link. I think it's spambot-proof.