You won't even need a set top box if your TV already has an ATSC tuner built in (just about every HDTV built in the last several years does.)
Yes, they will be free and in HD.
It looks like you get all four networks (ABC, CBS, NBC, FOX) pretty strongly, along with a couple of local channels and Univision. Those can probably be picked up with any decent indoor antenna. PBS, CW, MyNetworkTV, and ION might need an attic or roof mount. As Taperwood said, check with the HDTV technical forum for the gurus of OTA.
Don't believe everything on the Interwebz! A duck's quack DOES echo!