Provided you have online access, far and away the best way to find Wi-Fi hotspots is an online directory. The hitch, of course, is that you have to be online in the first place. These directories don't do you much good if you are wandering aimlessly around Podunk with your Wi-Fi laptop or PDA all ready to go. The moral is, do your research before you leave on your travels.
This section lists the best online directories. You can use them to find hotspots of all sorts (both free and for pay) in all locations.