what are some of the most common applications for diminished arpeggios? in a metal/rock context. I searched internet but cannot find any thing with a good explanation. I know these arpeggios have a flat 5 so how can it sound good over anything? many chords incorporate a perfect 5th and it would sound too nasty to play it over a power chord wouldnt it?
Diminished triads are mostly found in first inversion. Leading to I or Ib in a major or minor key.

In minor keys (and major keys when borrowed) the diminished triad appears on the second degree, and can be used in a iiob V i cadence.

Diminished 7ths are found on the 7th degree of the harmonic minor scale. Resolving to i.

You can play a diminished 7th over chord V which gives a 7b9 sound. You can also approach chord V with a #ivd7 chord.

These are the most basic applications of diminished triads and diminished 7ths. I'll post more later because I'm going out soon.
I have a ton of tutorials that show the many applications and concepts of dim7 arpeggio's, Harmonic Minor/Phrygian Domants scales, Diminished scales, etc...:

Diminished Concepts in Common Music - http://mikedodge.freeforums.org/diminished-concepts-in-common-music-t26.html
Copping the Jimmy Herring diminished vibe: http://mikedodge.freeforums.org/some-thoughts-on-copping-that-jimmy-herring-diminished-vibe-t6.html
Cop Some Of That Yngwie and Harmonic Minor Diminished Sound: http://mikedodge.freeforums.org/cop-some-of-that-yngwie-and-harmonic-minor-diminished-sound-t29.html
Navigating the Minor 2-5-1 and the Diminished Sound: http://mikedodge.freeforums.org/navigating-the-minor-2-5-1-diminished-arps-and-the-t33.html
Chord Progressions - http://mikedodge.freeforums.org/chord-progressions-t40.html (looking through this you'll see a bunch of uses for the dim7 chord/arp

Before you get to those first use little example to get the diminished sound in your head. The key to using it is to first HEAR where it sounds right. On it's own it doesn't sound like much but use where people commonly hear it as being correct is the FIRST place to start when using the sound. And, by learning some basics you'll understand and hear every place you've heard it before but didn't know what you where hearing.

One of the most common sounds is against a minor cadence like...

||: Am | Am | E7 | E7 :||

You can think of this scale wise as:

Am = A Natural Minor
E7 = A Harmonic Minor (or E Phrygian Dominant which is the 5th mode of A Harmonic Minor)

There are three dim7 arpeggio's in the A Harmonic Minor scale...G#dim7, Bdim7, Ddim7, and Fdim7.

These four arps are interchangeable because one, they ALL contain the same four notes, and two, because they ALL resolve back to the Am triad (this is VERY important in learning how to use them).

Ok lets apply this idea and test it out...

Since the dim7 arp is a chord and the E7 chord is too and they are interchangable, let's start by replacing the E7 chord with the dim7 sound...play through/string together these examples one after the other (I'm making them up randomly because they all have merit and you want to wrap your head around the freedom you have with these four dim7 chords in this progression):

Remember, play each repeat and then play right into the next progression as if it where one long progression endless progression...

||: Am | Am | E7 | E7 :||
||: Am | Am | E7 | G#dim7 :||
||: Am | Am | E7 | G#dim7 Bdim7 :||
||: Am | Am | E7 | G#dim7 Bdim7 Ddim7 Fdim7 :||
||: Am | Am | E7 | Fdim7 G#dim7 Bdim7 Ddim7 :||
||: Am | Am | G#dim7 | Bbdim7 :||
||: Am | Am | E7 G#dim7 | E7 Bdim7 :||
||: Am | Am | G#dim7 Bdim7 Ddim7 Fdim7 | Ddim7 Bdim7 G#dim7 Fdim7 :||
||: Am | Am | E7 | E7 :||

Hopefully that firmly planted the dim7 sound that's present in that progression and you're ears are now hearing the strength of how is resolves to the Am chord.

Now that you are hearing it, start experimenting and testing out the ride so to speak. Swap those dim7 chords around at will, take chances pushing it and trying to make the sound fail...chances are the sound won't fail, but you being able to stick them in freely will breakdown first. KEEP PRACTICING being able to freewheel this sound and you being able to pull off as many arps as you can in small a two measure pass. This will strengthen your playing and it'll give you confidence to lean heavy on this sound without any apprehension of it not sounding right.

And don't forget that the E7 chord can fit anywhere in those last two measure, so do not abandon that chord completely.

If you're to the point where you can manipulate the crap out of all of those chords...start substitution the four dim7 arpeggio's in your lead playing.

You can use these four dim7 arpeggio's as "chucks of the A Harmonic Minor scale"....so we can now look at things scale wise as:

Am = A Natural Minor
E7 = A Harmonic Minor, E Phrygian Dominant and G#dim7, Bdim7, Ddim7, Fdim7 arpeggio's

Now with this sound plant in your head start looking at those lessons I posted, they'll open your eyes to a TON of applications.