An origami envelope which looks almost like a classical glued envelope, but can be closed by folding only. If you use it for sending a letter by actual postal service, I would still recommend gluing the loose triangular flap just to be sure the envelope is not accidentally opened. This is a very simple design, so I wouldn’t be surprised if someone else invented it before even though I haven’t found an identical one on the web so far.
I recently got into origami envelopes since this is a practical use case for origami. And since both this one and some other simple designs can easily be modified for folding any tessellation into an envelope, I find this a nice way of trying out a tessellation design without having to fold dozens of molecules. In this respect, envelopes are similar to boxes: they make it possible to fold a mildly interesting model using just one or two tessellation molecules.