Not sure if this is the proper place to post this, and you could argue mockups count as screenshots. However, there is a difference. Mockups are essentially fake screenshots put together in an image editing program, rather than actual playable games. Don't often make these, however, I did make one today for Pixel Dailies on Twitter:
It's of Aqua Planet from Sonic Chaos. Not the most proud of it, but I tried at least. It's intentionally made a bit different from the original though.
Feel free to share any you've made too!