This is a simple and easy conversion that I know many chaos players like to make. Perhaps it is so obvious that it doesn't need posting, but I probably wouldn't have thought of it when I was a newbie to the hobby.
Ingredients: one marine backpack (chaos or loyalist) and two gargoyle heads from the chaos tank sprue.
Method: chop off the exhaust vents from the backpack and ensure that the remains are filed down to a flat-ish surface (likewise for the back of the gargoyle heads). Then simply glue the heads on to where the vents where previously located. [The image shows a plague marine with a gargoyle headed backpack].
A simple, easy and effective way to create variety in your squads. (Note that some loyalist chapters come with similar designs for free -- see the space wolves marine sprue for instance).