I may get skewered for throwing in my two cents here.
In the US you can refer to a website -
http://wheretowatch.org/ which is sponsored by the MPAA and lists legal sources for movies and tv shows in the US. Going by the wheretowatch.org website and not commenting on other aspects of legal/illegal the following are currently available for XBMC on some platforms:
For really current movies you need to pay a fee for Netflix, Amazon, Hulu, Vudu or a similar service. These are all supported in one way or another on XBMC, though some folks are unhappy with having ads, paying a fee or using an external player to use the service. Not all HW/OS platforms are supported.
For most of the current US TV Shows, the USTVVod addon (it's in the xbmc.org forum) handles nearly all the legal TV sources listed there as well as Crackle movies. There is also an addon for MeTV for a few older tv shows in the xbmc.org repo.
EpixHD has a few reasonably current movies and a library of older ones in HD. I'm not sure where the repo for this is any longer, it used to be in the BlueCop repo. The version I have still works after 2 years of use. You need to sign up for a free account which lasts about a month to use.
Youtube is, well Youtube and is available in a couple of addons listed in the xbmc.org forum.
For older, eclectic movies as well as some reasonably current tv shows you can use Viewster, Popcornflix or SnagFilms which are available in the xbmc.org repo.
There are quite a number of other legal addons available, but the above are a sampling from wheretowatch.org.