PS Now is a subscription service which allows you to stream PlayStation games to your PC (or your PS3 and PS4). Right now, a monthly fee gets you access to over 400 PS2 and PS3 titles – with many exclusives among them – but soon it'll boast an unspecified number of PS4 games, too.
This is good news for anyone keen to play any of the high profile PS4-exclusives in the console's catalogue: Uncharted 4, Nioh, Horizon Zero Dawn and um, Knack, count among them. Sony hasn't confirmed which games will feature in the PS Now selection, nor whether they'll necessarily be exclusives, but given the service's history they'll likely make it there eventually. Currently, the best reason to subscribe is access to Red Dead Redemption on PC.
Is it worth it? Depends what you want to play, but Joe gave the service a test spin last year and found that it worked decently performance-wise (his connection wasn't much higher than the minimum required speed to use the service). If you don't have a PS4 and must play a PS4 exclusive, it seems like a short term subscription could suffice.