My gut reaction is to flag the heck out of this answer:
http://ell.stackexchange.com/a/13144/50

However, I have to concede that the site being advertised is on-topic. (No clue about quality, as I have no wish to actually *visit* it.) Also, the post fully answers the question before it gets to the advertising, so it's not blatant spamming.

I don't know if I've encountered anything like this on any other site. I'm not seeing anything about link-dropping or advertising in our FAQ. (Aside: could we make the FAQ any harder to find? What's wrong with a link that actually *says* FAQ? Sheesh. But I digress.)

What should we do with an answer that includes advertising?

1. Nothing, as long as it's actually an answer, not just a link.

2. Edit out the advertisement portion.

3. Flag as spam.

4. Something else?