If you want your kids to be able to open the door when they are playing outside, you can put a rope though the mailbox for them to pull. If you have a round doorknop like I do, then this isn't really an option. Therefor I created this simple removable handle. It's removable because I don't want it to be on my door all the time. Let's say I don't want to make it very easy for people who are not my kids to come in. When removed only a small piece is left on the doorknop.
I think it should be compatible with most doorknobs since it's all pretty much standardized.
10-10-2015: Added improved arm because the original was not strong enough on the long term.
I would suggest using 3 outlines to make it a bit stronger. Print the 'nut' with 100% infill.