The following are links to sites which I have found to be useful resources for gaming.
Atomic Rockets – Lots of information on rocket and spaceship designs.
Though the following are mostly of use for Pathfinder, some are potentially more generic.
Pathfinder SRD – The main SRD reference we use for Pathfinder.
Random Name Generator – A random name generator for various cultures in Golarion. Could be useful for other settings as well.
Random Item Generator – Useful for generating random treasure hordes, and also lists of items available for purchase in a city.
For What It’s Worth – The actual cost of things in dark ages England.
Ancient Fonts – Unicode fonts for ancient scripts.
When making maps, it’s useful to have access to textures. These resources are actually meant for 3D modelling, but the seamless textures can be used for 2D maps as well. They are all Free, as in can be used in your own work, either commercially or not.
CC0 Textures – These are all available under a CC0 (Public Domain) license, so can be used without needing to give credit.
Texture Haven – Again, another site providing CC0 textures.
By free, I mean they can be re-used without permission or giving credit, so can be easily used on websites and shared work.
Piqsels – Public domain images
Pixabay – Stunning free images & royalty free stock.
By non-free, I mean they can’t be re-used without permission. So their use can’t be shared.
Klaymen Ojalvo – Some really nice portrait images, useful for character portraits.