Here's an interesting thingy - NVIDIA's latest GeForce Experience beta includes a new feature called GameStream Co-op, letting you stream a game directly to a friend and play it together. Or, if that friend's really less of a friend and more someone you actually can't stand, hand over the controls altogether and just let them get on with it.
The obvious application for GameStream Co-op is tagging in a superior player to help you tackle a tricky section or a boss fight - assuming the streaming latency doesn't provide too much of an issue.
But it's also an opportunity to sample a game you don't own yet, dropping in at any point to get a representation of it - we're long past the days of demo discs, after all. Much as this writer laments the passing of those days. (Remember the Half-Life demo and its bespoke content?)
Sony have a similar function on PS4 called Share Play, used for both online co-op and remote gaming. However, NVIDIA's solution requires quite specific hardware and software environments.
Firstly, the host PC needs to be running a GTX 650 or better (or a non-Optimus GTX 660M laptop GPU), a Core i3-2100 CPU or faster, 4 GB RAM, and an 802.11 a/g router with at least 7 MB upload speed. The other player doesn't need to meet those same GPU requirements but they do need to tick the other boxes, packing 7MB download speed rather than upload.
Currently streaming is capped at 720p and 30 FPS, and sessions require a new invite from the host after 60 minutes, so GameStream Co-op is unlikely to ruffle any anti-piracy feathers.