Just a couple suggestions for the xkcd link button.

Something like [xkcd=391]CLICK HERE[/xkcd] currently does exactly the same thing as [url=http://xkcd.com/391/]CLICK HERE[/url], and it's not that much shorter. I personally find it harder to use that [url][/url] tags because I'm generally ready to paste link when I want to link to something. I haven't really seen anybody use it either, so IMHO it's just cluttering the buttons row on the post page

On the other hand, if the [xkcd][/xkcd] mod could actually fetch the image on that page, stick it in, and link to the webcomic, that'd be awesome If that's possible, I'd suggest syntax like [xkcd]391[/xkcd] for it, similar to the Vimeo button.