Invaluable (if taking the tours) – Big Bus Tours Review

Used this app while in D.C., and had no issues with it crashing. The interface, however, is not the friendliest, so here's a pro-tip: use it a little before trying to figure it out on the fly. Great as a quasi-map for some of the sights, and got us right where we needed to be on at least two of the tour lines!
Review by {NE}^TheMilkman on Big Bus Tours.