Plastic welder: My new favorite tool.

I've used plastic welders with decent success.

An old trick my dad taught me: if you're lucky and the material you're repairing is ABS, take a utility knife and shave an old ABS pipe into a glass jar and add a touch of acetone. Don't breathe the fumes! Mix with a wooden dowel and voila! Use this "goo" to make your repair.

I've used this method to repair cracked radiator shrouds. Not sure if they were ABS, but it worked wonderfully.