A very wonderful website to visit that provides the best selection in movies is Netflix. Netflix is great. you can simply start by signing up and select which package you want.
Once you've done that you can watch the movies right away from you own computer. They also provide the service of having your DVD's mailed to your home to watch
Another site that I love a lot is Hulu. Hulu is great for watching not only current movies but tv shows as well.
The last sight that I use sometimes myself is Youtube. This site can be very useful to find tv show as well as watch some full length movies. Keep in mind these will not be very new movies but they can be very enjoyable.