Here are a couple starter apps from the growing Travel category, ranging from handy reference (train schedules and maps) to self-guided tours and media.
BEFORE YOU GO
HearPlanet: Audio Guide to the World
Rick Steves Paris: Historic Paris Walks, Lourve Tour, Orsay Museum, Versailles
Rick Steves Ancient Roman Tour
WHILE YOU’RE THERE
Urban Spoon – works in London
WHEN YOU GET BACK