It's hard to find a city that doesn't have some kind of discount attractions card these days. In the U.S., CityPass now offers cards in 10 North American cities, and on EuropeanCityCards.com, I just found links to 45 cities in Europe. (Though the site claims only 43 cities. Modest or just slow to update?) Not all city cards are included on the site—for instance Paris, which has the Paris Visite card, isn't listed—but dozens are.
When you're deciding whether or not it's economical to buy a card, make sure to weigh the number of attractions you're actually likely to visit (and remember, most cards are only valid for a few days), and then check entrance fees against the cost of the card. The second part tends to be much easier to do online before you go rather than once you get there. Generally, cards are best for Alpha sightseers and big museum-goers. If you're the type who might go to a museum or two before spending quality time in a local cafe, bar, or park, a card probably won't be worth the purchase price.