Netflix started rolling out its Watch Now feature in waves a while ago, and I just now was included in one of the waves. So, the deal is that you can watch a movie on the website for free. This apparently only exists for some movies (likely due to licensing issues). For example, only 21 movies are available in my queue of 431 discs (although to be fair, a good percentage of the discs in my queue are TV series).
The way they limit it is by the cost of your plan. Since my plan costs $18 per month, I can watch 18 hours of movies per month, which, given my current patterns, is more than adequate. I will probably mostly use it to fill in times when the physical DVDs are in transit, but I imagine I'll also use it when trying to find something to watch with other people. I'll probably try it out this evening.