A few other options:
QB: In addition to Moore, Jason Campbell, Tyler Thigpen and Rex Grossman are all upgrades over Leftwich.
RB: A lot of blech. Jerious Norwood maybe as a change of pace type back? Goodson runs well when healthy but seems to always be hurt.
WR: I've always liked Greg Jennings, but he is all but sure to be out of the Steelers price range and has had injury issues the past few years. Sadly, if you need a free agent WR, the best one in the class is Mike Wallace.
DT: Terrance Knighton - young run stuffing DT, likely too expensive.