The thrusters use hydrazine. It reacts with a catalyst and decomposes into hydrogen and nitrogen. They are used to steer, so the fuel isn't used up.

http://www.slate.com/articles/news_and_politics/explainer/20...

They are pressure fed hydrazine monopropellant thrusters. They are incredibly reliable. Keep in mind that Voyager 1 doesn't have reaction wheels or any other method for controlling its attitude other than its thrusters, and it's 3-axis stabilized. The original thruster set fired thousands of times over a nearly 4 decade period and now the backup thruster set is being rotated into use.

All the references I found Wikipedia said it used Hydrazine thrusters.

As to why there's propellant left: it's a sealed system and NASA has intelligent engineering practices when it comes to resource planning.

I always thought "thrusters" were devices that always spewed out majestic flames and plumes of smoke whenever they are fired. I was surprised, as a kid, to learn that the Voyager thrusters effectively are no more exciting that pressing the nozzle of an aerosol spray can for a split second. Stands to reason that they don't need much force to affect the spacecraft inertia at those weights and speeds.

Still, a stupendous achievement to have them still working 40 years after being deep frozen in space!

Irrespective of what is used, consider that in the vacuum of space, when you're not in a hurry, you'd only need the tiniest bit of propellant/thrust for an attitude correction. Even for a comparatively heavy vehicle like voyager. The other forces at play are so miniscule, that you only need a ridiculously small force to start it rotating (and the opposite when you want it to stop).

propellant, and because they didn't use it all.

Hydrazine