Water, Water, Water
Not Quite Everywhere
If you are by the ocean, it is possible to use tidal flows through narrow channels or use the action of waves. On a grand scale one could create very large passive pumps to draw water from the depths to the surface and use the temperature difference. Or one could create huge funnels and tap the slow-moving open ocean currents. If you are near a river flowing into the ocean, you can use the difference in the salt concentration between seawater and river water. Two practical methods for this are reverse electrodialysis (RED) and pressure retarded osmosis (PRO). Both processes rely on osmosis with membranes.
Of course, there are lots of places far from the ocean, where the rivers are just too small, or where water is scarce and not really useable for power generation. Even being near a large lake doesn't really help for generating power unless you happen to be where the incoming or outgoing rivers are located.