I recently built an Android app [1] that implements this idea in pretty much the most basic way possible. It just fires a notification at a specified time every day. The notification can link to a URL, which I point to a Google Form for responding to the notification (e.g. "How many pushups have you done today?"). I've been meaning to allow push frequencies other than once-daily, just haven't gotten around to it yet.
[1]: https://github.com/meshulam/PushQ