They have a Squeak No More Tool, that works wonders for floors that squeak under Carpet, expecially on the Second Floor on Homes - You Screw from the Top Down into the Joist and Snap the Head of the Speacial screw off and you never feel it.
For Sqeaks under Hardwood, Tile, Vinyl, Etc (anything other than Carpet) It is called "The Squeak Ender"
You can pick up Both tools at Home Depot. The Sqeak Ender fastens from the Bottom of the Floor Joist. It screws into the Subfloor and then brakets itself to the Floor Joist with a Wrench, you can Tighten the Bolt down until it pulls the Subflooring back down to the Subfloor.