The FlyingBlue engine on airfrance.us is the most comprehensive free tool. It searches many partners, but is generally worthless for US-Asia itineraries using SkyMiles, as it tries to route via Europe. (DL generally doesn't allow this.) KVS will claim that they're the best, because they have inventory for all SkyTeam carriers. In reality, they spend a lot of time screen scraping the FlyingBlue engine and DL.dumb plus pulling a few extra things from a GDS or two. Wandering Aramean makes a few things available for free that you won't find on FlyingBlue (SU, for instance). He also had KE inventory, but last I tried it, it was broken.