SRAM Tire Pressure Calculator
Looking for really specific tire pressure? Try using this nifty tool.
Otherwise keep reading below for some good tire pressure guidelines.
Tire Pressure for Road and Gravel
Good tire pressure hinges on a lot of different factors. How you ride, where you ride, when you ride, what the weather is doing -- these can all have a pretty dramatic effect on what you should be doing with your tires. The best advice is always to start off on the hard side and gradually lower pressure until you've found something that feels like it gives you the best combination of grip, precision, speed, and ride quality. Those are all pretty subjective concepts, too, which makes trying to tell you what pressure you should be running just that much trickier.
Luckily, we've got some firsthand experience and access to just enough science to make us sound like we know what we're talking about. The chart below is a good place to start. 
One last point to mention.
These numbers are based on tires with innertubes. You can (but don't always necessarily want to) run your tire pressure a bit lower with tubeless setups (5%~10% for tires up to 32mm, 10~15% for 35mm and wider).
We say you won't always necessarily want to run lower pressure for a couple reasons. For one, bad roads with potholes and rocks can be a real hazard to your rims. Dented alloy rims or cracked carbon ones can create a gap big enough that your sealant won't be able to heal it. Running a little higher tire pressure in situations like this can prevent those hard hits from turning into hard lessons. Secondly, the ride quality will still be better and the rolling resistance will still be lower, even if you run exactly the same pressure without tubes as you did with. This is because the elimination of the second layer of material and the extra internal friction caused inside the tire by the innertube lets the tire be more supple. In fact, it could be argued that this is the most significant factor in the improved ride quality and performance of tubeless tires. But you could also argue that Pluto should still be a planet. Some of us just need something to hold onto, okay?