CinemaNow is a movie and TV streaming service owned by Sonic Solutions and co-branded by BestBuy. Movies and shows can be purchased or rented, then downloaded or streamed to a computer or an Internet connected device like many Blu-ray players and TVs.
The selection is good, though there is far less HD content than the other competing services. Blockbuster on Demand is essentially the same service.
For a history of CinemaNow, check out the wikipedia page.
HDGuru.com has a list of what products offer CinemaNow.
Competing services are Amazon Video on Demand, Apple iTunes, and VUDU.