I think this was my favourite hot pool in Iceland. Pretty well in the middle of nowhere, near a used-to-be hotel, was this little gem. To get to the hot pool, you drive through Búðardalur and continue up Road 60 until you reach Sælingsdalsvegur. It is just after side road 590 on the left hand side when you are heading north.
The pool is completely free to use, a pleasant 100°F (enough to warm you up but not as hot as a hot tub you'd have at home). Of course, always respect the environment when you are there by packing out whatever you pack in and if you can, remember to not use heavy lotions or harsh soaps the day you use the pool as you could contaminate the water for other bathers. It was very clean when I visited and I had the place entirely to myself. I believe since it is a little bit out of the way, there are much fewer tourists here than most hot pools.
In short, I would highly recommend Gudrunarlaug if you are driving the ring road.
For Google Maps coordinates to the parking lot click here.