I use and recommend Toggl to anyone and everyone, even more so now that they have added support for Phabricator tasks. Anyway, Toggl is a time tracker that is built for speed and ease of use. With Toggle, time keeping is so simple that timesheets are no longer a dirty word.
Why are they app of the week? Well here's the thing - I've been using their apps for a while now, and it turns out that the source code of their Android and iOS applications are open-source on GitHub. Additionally, the entire roadmap/development is done out in the open as well. Finally, both apps are-developed with Xamarin and MvvmLight!