Every time I travel I refresh my apps designed to be used when the iPhone is offline. These apps all cache data so I can use Wikipedia or a map without a WiFi or cellular connection. I started doing this because international roaming data was so expensive but the apps are now good enough that I think I will use them even when I’m home. Cached data = fast! Here’s the best of the lot, I believe all these apps are available both for iOS and Android.
ForeverMap 2: OpenStreetMap. Download a few hundred megabytes and have a map of a whole country in your pocket. Routing too! Map data quality varies based on OSM coverage (it’s great in US and most of Western Europe). The rendering and usability of the app is fantastic. They also have a turn based navigation program I haven’t tried. I’m amazed Apple hasn’t yet bought Skobbler to help fix their maps problem.
Wiki Offline: Wikipedia. Download 4GB of English Wikipedia once, read forever. The formatting is finally good enough that most articles come through unscathed. Only thing missing is the pictures. Being able to wikidive without waiting for network is terrific.
Triposo: travel guides. Triposo scrapes open data sources like Wikipedia, Wikitravel, OSM, and Flickr and then compiles it into a usable offline travel guide on your phone. It’s great for answering the question “what are the three things I should see in this town, and where should I have lunch?”.
Ascendo dictionaries. There's a zillion low quality free translation dictionaries out there, this one seemed to have a decent German database and work well offline.