The best way to find free wifi overseas is right here, in planning your trip. They are hard to find unless you have a prepared list. As I recall, the main square in Tallinn, Estonia had freel wifi. Estonia is a pleasant country to visit and the only one I know of that has a picture of a chessplayer (gm paul keres) on one denomination of its paper money.

The most popular place to use wifi everywhere is at home, so in theory there is lots of cheap wifi available. Yesterday I encountered an intriguing site, www.couchsurfing.com, dedicated to sharing free lodging for travelers worldwide. You could use contacts from that site to line up free places to use wifi from, I imagine.