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.
China Pulse: www.chinapulse.com/wifi (hotspot locator for China)
i-Spot Access: www.i-spotaccess.com/directory.asp (currently limited to Illinois, Iowa, Missouri, and Nebraska)
Ordinance Survey: www.g-intelligence.co.uk/wireless/wifimap.php (great for United Kingdom hotspots)
Square 7: www.square7.com/hotspots (great for European hotspots)
Wi-Fi-Freespot Directory: www.wififreespot.com (focuses on free as in "you don't have to pay" hotspots)
WiFinder: www.wifinder.com (one of the best all-round international hotspot directories)
Wireless Access List: wirelessaccesslist.com/wireless/u.asp (categorized by state and ZIP code, also allows sorting by network, for example, T-Mobile, Wayport, and so on)
Zagat Survey: www.subscriberdirect.com/the_new_yorker/zagat/(lists and reviews restaurants and hotels with Wi-Fi access in five major U.S. cities)