According to an article in the September issue of Fast Company, Kayak.com is the only site that lists fares on Southwest. Well, uh, no. Kayak does not list fares on Southwest. Southwest appears as a link when you search for a fare on a route they serve, but the Kayak link just leads you to Southwest.com, without giving you a fare for the route you searched. You're on your own when you land on Southwest's site.
As you know, Airfarewatchdog.com is the only fare listing site that includes Southwest, and that's because we research for their fares the way any consumer would; we don't use software or "scraping," which was what Kayak attempted to do, until they got that cease and desist. You're on your own when you click on our Southwest links, too, except that we've at least told you what the lowest fare we found is, whereas Kayak doesn't even do that (they just tell you that Southwest flies the route).
Anyway, Kayak also doesn't list fares on American anymore, either, although a note on their site says they hope to have the "issue" fixed soon. So Kayak now doesn't list fares on the two biggest US airlines based on domestic passengers carried.
And while we're at it, Kayak, could you please stop with the Priceline-checked-by-default nonsense, so that whenever we do a search a Priceline pop up window appears on our screen? This is not a good user experience. Yes, we know, you've got to make money, and although we don't know how much Priceline is paying you for this, it's just not worth it, in our humble opinion.