There are a lot of common place conversion techniques that are often found. Most of these techniques involve a head and weapon swap from a Cult troop.
-To make Berzerker Terminators, replace the metal head with the head of Kharn or a regular Berzerker head.
-to make Plague Terminators...use the heads with tubing from the PMs.
-1K Sons...again, a head swap or use Ahriman's head
-Noise Termies...no clue, but there are some good guides on the Slaanesh yahoogroup list. I'll try and pull some for you.
Those are the easy ways, but I recommend finding your own method. I currently just use regular Terminators, but I plan on converting a squad from the Space Marine Terminator Chaplain. That skullhead is hard to pass up!
There are all sorts of cool things you can do...but first you must decide on what it is you really want them to look like.
When it comes to conversions...anyth ing is possible.