http://mapswith.me has become very popular recently. I haven't used it much myself so I can give you a proper comparison. Otherwise, OsmAnd is open source and can be found in the F-Droid repository.
I'm giving mapswithme a try right now. I love that I can download all of the maps that are relevant to me, and then use it without an internet connection.
Because its maps are offline, I've used it on a plane to track flight progress