I’m honestly not sure when it debuted. Could have been last week, could have been six months ago. All I know is, I didn’t notice it until now. Maybe you didn’t, either.
The feature is hiding in plain sight. When you look at the listings for a particular category or the results of a search, you should see a link just above the listings that says show images:
Click it, and what do you know? A thumbnail image appears alongside each listing (where applicable). And if you mouse over that image, you’ll immediately see a larger version:
This isn’t perfect. The enlarged images aren’t large enough for my liking, and the preview shows only one image, even if the listing has several. But it’s still a major time-saver and a very easy way to preview what’s “beneath” a Craigslist ad — no browser extension required.
It probably goes without saying, but if you want to turn off this preview, just click hide images.