The new feature may or may not be a response to TripAdvisor's new airfare search, which was launched with a similar "fees estimator" tool a couple of months ago. To figure out the bottom line for what you'd pay using either tool, all you have to do is select the number of bags you're planning on checking, and updated flight prices with baggage charges included appear in seconds. While Kayak's tool is limited to baggage fees, TripAdvisor's will also give a rough estimate for on-board charges if you're interested in things like alcoholic beverages, food, and headphones. At this point, however, so few airlines still offer these amenities for free that travelers can assume they have to pay for pretty much anything above and beyond a seat.
For now at least, the other big booking engines aren't incorporating baggage and other fees into their flight searches. That's not to say the other sites are leaving travelers totally in the dark, though. Travelocity, Priceline, FareCompare, and Orbitz keep airline-by-airline lists of who charges what for checked bags. The latter two (FareCompare and Orbitz) also give estimates on how much snacks and meals cost on each airline.
While it's nice to be kept informed, all of these fee tracking services should serve as a reminder: Whenever possible, travel with a carry-on only, and bring whatever food and entertainment you'll want on board the plane. If you don't, you can expect to pay extra.