Truth be told, the majority of the free apps I'd downloaded were useless during my visit to London. But there were also a few amazing ones that really enhanced my trip. They're all available for Android through the Google Play Store, but I think each has an Apple version as well.
Best of all, all of these tourist apps work OFFLINE! Be sure to bookmark this list for your next trip (and if you don't have a trip to London planned, start planning one. It's the best place I've ever visited!) Here are my favorites, in order starting with the one I used most:
Top 5 Free Travel Apps for London
1. London Tube Assistant
I downloaded every tube map, tube tracking, and tube route planning app I could find and this was my favorite, hands down. Before venturing out on my own, Sexy Nerd tried to teach me how to navigate "the tube". You just look at the map, find your station, find the station you want to get to, try to find a tiny colored line that connects the two (and includes a stop, as opposed to just passing through,) check the key to see which line that is...blah, blah, blah... This app made it so much easier! You just tell it where you want to go and it gives you different options for getting there. This app turned me into such a London tube pro that I by the end of day 1, I was already giving lost tourists directions and was running down the left side of the stairs to catch my next train with the locals (and cursing out the tourists who stood in my way. Stand to the right, folks!)
2. London City Guide from TripAdvisor
TripAdvisor's handy app saves all kinds of information, from restaurant reviews to details on popular attractions, for you to use offline. There's a map that uses GPS to show your location in relation to where you're trying to go and it syncs with your TripAdvisor account. There are suggested itineraries. It can even tell you what's nearby so you don't miss anything!
3. London Travel Guide by Triposo
This is a little-known app that enriched my travel experience. It includes a currency converter (though it's a depressing feature to use if you're converting US dollars to British pounds!) and, like my beloved TripAdvisor app, it has suggestions on where to go. However, what I enjoyed most about Triposo's app were all the educational tidbits about the places I was visiting, taken from sites like Wikipedia. My meandering took me to Somerset House one day. I didn't know anything about it, but the app filled me in on all its interesting tidbits, including its impressive film history (per my Triposo guide, Somerset house has appeared in Tomorrow Never Dies, GoldenEye, Sherlock Holmes, and Sleepy Hollow, among others.)
5. The London Pass
Although I didn't purchase a London Pass during this trip (I've bought these passes in San Francisco and Seattle, but didn't think it was worth the money for London due to all the activities available for free,) I still enjoyed their app. Like the TripAdvisor and Triposo apps, it was another convenient reference for London's attractions. I loved that it included photos, prices, and operating times.