Gmail and GCal for being able to manage email and appointments pretty much wherever I am. Evernote for having a portable electronic filing system that syncs between multiple devices, operating systems and apps. Dropbox for syncing/transferring files. PDF It for saving web pages (the one app that keeps me from switching to Chrome). And finally, if mobile phone apps count, MetrO for world-wide offline (= no data charges) public transport route planning. I’d be lost without it.