Close to home, your choice of ATM is likely to be heavily influenced by whether you can use it for free. Overseas, you'll probably get slugged with a fee no matter which machine you use, so you might as well opt for convenience. ATM Hunter helps you find the nearest ATM by entering an address or using your GPS.
Nick highlighted ATM Hunter, which is developed by Mastercard, as the best option for finding ATMs in Gizmodo's excellent iAppalooza series. You should still apply some common sense to your choices — ATMs at banks will potentially cost you less than those in convenience stores, but the latter might be a safer place to withdraw cash later at night.