Pstring talked about the normal powerchords used by most metal bands.
Now I like to add a more dark sound to my powerchords sometimes. If you ever listened to Death metal you know those "powerchords".
Instead of doing (on the 4-5-6th string)
---7
---7
---5
Try
---7
---5
---5
This gives a much darker sound. It probably has a real scientific name or something but I don't know it.