For scales what you'll find the most useful will be the major and minor scales across the fretboard and where they are in relation to each other (for example, G major has the same notes as E minor).

The chords used in metal are mostly just power chords. With dropped tuning they're just a barre across the lower strings.

Normal tuning F#-
244xxx
Dropped to D tuning F#-
444xxx
Tuning can also be dropped lower, but then the other strings must be dropped down to compensate. For example, Disturbed drops to C#. That is, the E string down 3 semitones and every other string down one, C#,Ab,C#,F#,Bb,Eb.
Dropped to C# tuning F#-
555(0)xx

I personally play small two note chords by ear just as much as normal power chords when writting riffs. For example (I wrote this riff in a wierd rhythm and I don't know how to write it out, so play it however sounds good to you).


a-4-4--7-7--6-6--2-2h4-|-4-4--2-2h4-5-5--2h4-----|
E-5-5--4-4--4-4--2-2---|-5-5--2-2---2-2--0-0-4h7-|

I wish I had some way of recording..arg..there are a lot of harmonics in the riff but I'm not tabbing it out for now for simplicity (and it's outside the subject).

What helps develop an ear for pitch depends a lot on the person, I don't know what will and won't work for you. Just playing a lot and getting used to hearing your gutiar is a good place to start. Something fun you might try that helps a lot of people is find a fairly simple song and try tabbing it out. I already mentioned to imitating vocals thing, the more you work at that the easier it gets.