Last year we made some community-promotion ads to run on other sites. I don't know how successful they've been in drawing traffic (obviously we still have growth challenges here), but it seems to me that where we haven't received negative feedback, it's worth continuing to try.
Last year we tried both general and question-specific ads, the latter intended to hook people with a compelling question related to the other site's scope. That got mixed results -- on UX (where we got the idea) the ad got a lot of upvotes and complaints from one person; on The Workplace the ad is live but has gotten more negative comments (including from the same guy as on UX, so maybe he just doesn't like us). Maybe we haven't found the right question/site combinations, or maybe we need to rethink the approach at least somewhat. If we can find different specific questions I think it's worth trying that again on UX, but maybe Workplace needs a different approach.
I don't remember if any of our current ads link to specific tags, but that's an approach I've seen elsewhere. So let's take a look at our tags and think about whether any of them have obvious fits with other (graduated) sites.
All that said, what ads should we submit to other sites? Please use answers to make proposals. Ultimately we're going to need art -- 300x250 pixels, which is a new size requirement -- but you don't have to wait until you have art to propose something.
Community Building is a small and underused site, but we're high-quality. Let's figure out how to present that quality elsewhere on the network in a way that encourages people to join us.