'To Do' Notes app?

Something i thought of while making a note to myself the other day is the 'to do' function found in Mail on a mac. why can't you type a note to yourself and set a reminder? just like with SMS, email, voicemail, etc, a number would appear in the Notes icon. when you completed the task you check it and it shows it's done.
i guess you could type up something in the calendar, but the notes app is just so much easier.
why hasn't someone thought of this app yet?

 

is it for your mac or for your iphone?
where to the 'to do' tasks appear on your phone?