My digital paper, checklists, project support and someday/maybe ideas are in Evernote, and my calendar is Google/Gcal, so I love zendone's tight integration with both.
Until this week it was in beta, and therefore free to use via web and iPhone. No Android app yet. Paid plans took effect this week, but there is still a free trial available.
All zendone does it GTD, so no elaborate setup is required - it is quick and easy to use.
But all it does is classic GTD, which can be a bit limiting