GeForce Experience is actually pretty popular; 65 million GeForce card owners have installed it, it supports over 250 games, and has enabled 250 million different driver downloads and installations automatically. It's getting a hardcore overhaul that not only lets you share your gameplay videos directly to YouTube or Twitch in-game, but also lets your friends control your games remotely.
The big announcement is gameplay sharing. Nvidia's ShadowPlay has already previously made it relatively easy to record your gameplay or stream directly to Twitch, but game sharing -- through a new in-game overlay -- makes it possible to enable Instant Replay (which records a buffer of recent gameplay, defaulting to five minutes), Recording (which archives it for later), Stream (which used to be called GameStream) or Broadcast (which lets you choose a streaming service, turn on the webcam, set video quality, and so on) from within games themselves.
These features have existed for a while, but there's now sharing features for each -- for example, if you have a recent Instant Replay that you want to share, you're able to upload it directly to YouTube after making minor edits (like trimming and setting a start and end point on that video), accessible through the same overlay interface. You can even edit your longer recordings and share those through the same interface.
Stream is the new name for GameStream -- Nvidia's existing device-to-device gameplay video encoding protocol -- which works over the 'net or over your wired or wireless local network. Stream will send out an email to whichever party you want to share your gameplay experience with, and as long as they have Windows 7 or above, a Core i7 or equivalent processor, and Google's Chrome browser, they'll be able to play as if they were sitting in front of your machine (albeit with whatever latency the network imposes, obviously).
Another feature enabled by Stream is local multiplayer gaming, through a sub-feature called Play Alongside Me. Demonstrated on Trine 3, Nvidia showed 720p30 streaming of the same game of Trine 3 playing on a desktop machine with one player controlling a character locally, and one playing via a separate laptop, streaming the game over GameStream, controlling a secondary character. Any game that supports local co-op will support remote controllers over Stream.
What you could do with Stream, Nvidia seems to be subtly suggesting, is that you might be able to share your just-purchased games with your friends, to give them a hands-on live demo. Games publishers might not be so happy about this, but they also might welcome any chance to show off their titles to anyone that wants to see them -- a bit of a test drive over the 'net, if you will.
By the same token, you could stream your game to a mate and have them beat a difficult part of the story for you. It'll be out "in September", according to Nvidia. [Nvidia]