TV reviewers often get advanced copies so they can write their reviews before the season airs.
Movies really aren’t leaked that often, most pirated movies still come after the release and high resolution versions usually don’t come out until the DVD release either.
What I’m more surprised about is how they manage to keep the results of reality tv shows so tightly under wraps for the weeks or months before it airs.
Up until the 6th season, Buffy was sent out to the WB/UPN affilates via unencrypted satellite and fans with C-Band dishes would promptly post episodes to Usenet up to a day before the Canadian broadcast. 
Starting with the 7th season, the lead time was reduced, to the point where you might be lucky if there was both time for your Usenet server to receive it and to get it downloaded to your PC before the episode actually aired.
One week during the 6th season, they accidentally sent out the episode “Seeing Red” a week early. That’s the one in which Tara died. 
I had that puppy burned to VCD (this was a bit before everyone had DVD burners) and it was a big hit at our sci-fi club meeting the following weekend, still 3 days before the episode was officially scheduled to air.
In some cases the results show is live so there is no way to leak the final result. I believe Survivor works that way.
I saw the unaired pilot for an Aquaman series shot by the makers of Smallville. The show wasn’t picked up and there was no way for anyone to see it, so of course it was “leaked” to the 'net. It wasn’t a bad show. I’d have watched it.
Someone mentioned Caprica, also “leaked.” Sometimes a show will be canceled with unaired episodes making their way to the 'net.
I see DVD-quality “pre-airs” leaked all the time these days. Here’s an article talking about it. Leak, or viral promotion? MHO is, at least often as not, leaks are viral.
Slightly off-topic, but Australia puts out some pretty good TV sometimes. Here in the States we’re lucky to have BBC America, but when are we ever going to get an Aussie TV America channel? A hot Australian show might get 2-3 million viewers, but if I were an Australian TV show producer I’d love to be able to go to the network execs and say, “sure we only got 1 million viewers here, but look at these torrent figures: 20 million viewers worldwide, with 10 million in the US alone.”
The only thing leaked with passing regularity is pilots, some of which were mentioned above, or first episodes of new seasons of existing shows.
The reason for this is twofold. With pilots, they are usually made well in advance of the TV series going on-air. They’re usually made before the show is even “picked up” by the network, they’re kind of demo reels for a showrunner. They make the first episode for a set amount of money, and if the network is tickled by it, they ask for more episodes. Because there can be a long lag between the pilot being made and anybody having even heard of a show, there’s more time for illicit distribution.
The second reason is sort of a product of the first. When new shows are announced, networks usually send out DVDs to TV writers/bloggers with the pilots of the new shows, hoping the writers will speak favorably about it and drum up bigger numbers for the show’s primiere.
They also do this sometimes with the first episode of new seasons, so writers can see or talk about the direction the new season is going in. This seems to happen more with shows where each season is its own story, like Dexter.
Because these DVDs are being sent out to people so unscrupulous as TV “journalists,” it’s not surprising that they somehow end up online.
When it’s about time for new shows to come on, if you search the torrent sites for “preair” you’ll usually find a lot of them.
If the networks actually have a problem with this, they’re complete fools, as all it does is serve as publicity for a show. I based my decision to watch The Sarah Connor Whatevers because the pilot I saw online wasn’t entirely awful (only mostly awful). I stopped watching after about 2 episodes, but at least they had me for those two, and they wouldn’t if I wasnt able to watch the pilot early.
Sometimes this can be interesting because sometimes actors are replaced after the pilots are filmed, so when you watch a leaked pilot you can see an entirely different person as you see later on TV. This happened with the pilot of the show Reaper, which had a different… whoever the main girl is, and True Blood, where there was a different actress for Tara (who seemed less annoying, IMO). I feel bad for the directors who have to go back and re-film entire scenes of their pilots because someone in a suit didn’t like one person in them.
Famous example of a pilot not having the eventual star was Star Trek where Shatner was not in the pilot. They later recycled the pilot by putting parts into 2 episodes - the ones with Captain Pike going to Talos IV. Jeffery Hunter played Captain Pike in the pilot and ironically he died at age 42 the same year Star Trek was cancelled - 1969.
This is true.
Plus, in the past they often printed out season/series finale scripts in dark red ink on red pages so it could not be photocopied or faxed by someone looking to earn a fast buck or just let their friends know the ending.
Also, they will sometimes call in lead actors to film “additional” scenes weeks or months later that not even the rest of the cast knows about - and thus cannot spill the beans, even if they wanted to do so.
In the case of some soap stars, even they don’t know they have been killed off until the day of the shoot - it is then that they discover they should have had a better agent and not pushed for that raise.