Award Nexus (www.awardnexus.com) can search all three alliances (each through one or two member airlines), plus a few independent airlines (who happen to partners of one of those airlines it searches), and it's free up to a point for FlyerTalk members (click "more signup options" under the green Purchase button to get to the place whee you can sign up for free using your FlyerTalk login).

But it doesn't show prices, since the prices depend on which of the alliance's member airlines you're booking with, and each alliance has lots of airlines, and every airline has different award pricing (in some cases dynamic award pricing, so you can't even use a chart).