I use a rock.

Not necessarily this particular rock - any from my collection of nice rocks will work.

Features include:

• free, you can simply pick up any larger rock
• flexible, you can put it anywhere
• it is a decoration when not in use
• looks very nice to a particular kind of person
• it punishes misbehaving children as they trip over it on the floor

(While I'm dead serious about me using a rock as a door stop, please follow up on what you end up using. I am invested now, after taking the time to take a photo of my precious rock.)

Well now, that is certainly a nice rock.

I have several doors that are challenging to hold open, usually for one or more of these reasons:

• they swing out over open air, so methods that rely on contact with the ground (rocks, floor wedges, kickdown stops, etc) won't work; and/or
• they open to the building exterior, so exterior hardware must be weatherproof (magnetic stops, chains and hooked stops, etc); and/or
• they face to the west, and the westerly sea breeze tends to overpower doorstops and slam the doors shut (small rocks, magnetic stops, etc).

I built a shed and installed "Stop and Prop" stops on it, and have been satisfied with them so far. They aren't actually visible in this photo, but they do a good job with the double doors:

One of my more challenging doors opens out into the exterior over a large step down to a deck and faces west. But I discovered the deck was significantly rotten and will likely hold this door open partially by rebuilding the deck to be closer to level with the bottom of the door, somewhat mooting this whole exercise.