I think it's mainly power chords (root, 5th, octave). The few songs I know of Foo fighters are played in dropped D tuning and uses power chords but sometimes with the open D as a pedal tone.
There are a few slower tracks which use the open major/minor chords with the occasional 7th thrown in for flavour. They are also a fan of playing a chord of just root and octave (not sure what to call them?) and running that up and down the scale, like in Everlong for example. It's fairly simple stuff but what I particularly like with Dave Grohls playing and writing is his rhythm. As a guitarist I think he still thinks like a drummer.
Hope that helps
Darren