Triposo Travel Guides have guides for various cities around the world. I first found out these guides in a post Andrew Fraser wrote about his trip to Italy. These travel guides all contain information and maps of the relevant cities that is stored on the device. You can use gps to see where you are on the map as well. Until the mobile networks make roaming charges realistic I think this is a great series of apps. I have included a few shots of the the Lisbon app in action below.