The Sopranos/WMG

Tony is Schrödinger's Cat.
Obviously no one knows what happened to Tony, including the people who wrote it. Obviously, that is something intentionally left unknown for a potential sequel, or if there isn't one, it will just be unknown forever.

Tony Soprano was neither killed nor arrested and lives out a full life in freedom.
The final episode contains no particular indication that Tony is killed or that he is in any greater danger of imprisonment than in the earlier episodes -- except that it is the final episode, and so we expect spectacular events. Oh, and the screen suddenly goes black and the episode ends. But remember what a large segment of the audience, including the head of HBO, thought about the blackout. They thought that their cable had failed and the episode was continuing without them watching it. The show's creator intended this, not as a practical joke, but as an artistic statement. By simulating a cable failure and getting millions of people to imagine Tony's story playing out unobserved, he avoids having any ending at all. After all, if you can imagine a few minutes of Tony's story that you're missing, you can just as easily imagine decades more. Even the soundtrack supports this theory. The movie never ends, it goes on and on and on and on...

The Sopranos and Max Payne take place in the same universe.
The Invincible Russian was on V, most likely the same kind that Max is on (see the Max Payne entry for more details).

The cameraman got whacked in the finale.
(This one was floated in another message board, but it's been altered a bit with additional guessing.)

The Sopranos, in their universe, have agreed to a reality TV show as part of some kind of plea deal. Because the FBI is looking for bigger fish, they turn a blind eye whenever a brutal murder happens (it is, of course, still good for future blackmail). But Tony has gotten increasingly upset at how he's been edited to look bad, and he's starting to cease to care about his supposed blank check from the government to do as he pleases as long as he has zero privacy and everything goes on the public record. So, while the family is eating in one of "his" diners, he decides to end things once and for all. The camera did keep running after this and got all of the murder, but the executives blacked it out because it was one of their own that was killed.

Tony and his Family were killed by terrorists.
The show dropped several hints and the potential connections between terror and the Mafia. The middle eastern men why try to buy TEC-9's from Chris are one example, but there's also the arc where Tony tries to buy his way out from under FBI scrutiny by trading Terrorism information. And then, of course, there's the final scene with the FBI agent where he worries about chatter of an imminent hijacking at Newark. Then, as Meadow crosses the street to enter the diner, the last sound you hear is the sound of an aircraft engine. The terrorists that tried to buy guns from Chris ultimately attempted their hijacking, but before they could reach Manhattan, something went wrong and the plane hit New Jersey instead. Obviously HBO couldn't show this ending in detail, because its still Too Soon.

Tony got whacked in the finale.
The entire show has been from his perspective. If Tony dies, then there's no show. The camera going black was him dying, thus taking away our view into his world.
 * As suggested in the line "...it's always out there. You probably don't even hear it when it happens, right?" Tony didn't.
 * David Chase has given at least one interview where he states that this line is the key to understand the final scene.
 * Of course this line could just as easily mean that Tony DID die, just not at the hands of anyone else. Fat guy, health problems, pushing 50, eating greasy onion rings in a diner with his family has a heart attack and dies. Wouldn't be the first time.
 * Tony's execution was foreshadowed earlier in the episode when Phil Leotardo was shot in front of his wife, who freaked and let the SUV containing their infant grandchildren roll over his head. The only possible response from New York was to take out Tony in a similarly public and bloody way all over his family, thereby tying off the two threads of his life.
 * Except that before Phil's murder, we see Tony and what's left of his crew discussing the situation with members of the Lupertazzi family who agree that Phil has crossed the line and essentially tell Tony that they won't help him find Phil, but they won't do anything to stop him either.

Tony was a cooperating informant for the government the entire series
Think about it: 1) All of his opponents ended up dead or in jail.ALL OF THEM. 2) He killed at least six people over the run of the series and was never brought for any kind of questioning or brought up charges. 3) The FBI searched his home and yet found NOTHING. No cash,guns,evidence,etc. 4) He was the head of a Mafia family ,was able to go to a psychiatrist for six years and wasn't whacked by either his men or his enemies.

How was this all possible? Tony was a cooperating informant and was feeding the government information on his crew and his rivals.


 * Y'know what... This troper quite likes this interpretation. We get a cut to black at the exact moment that 'The Sopranos' vanish into witness protection or similar, and become 'The Johnsons' or whatever. The Members Only jacket guy is in fact a Marshal or FBI agent there to watch over them before handing over new ID documents and moving them off to the future. The whole thing about Tony dying was that its such an obvious ending to the series, and its much more interesting to imagine ANYTHING else happening.

Carmella and A.J. are killed along with Tony.
It has been pointed out elsewhere out there in internet land that Tony, Carm, and A.J. popping those onion rings in their mouths is highly suggestive of taking communion. Assuming that Tony is shot in the head at the moment of the cut-to-black, then the killer is standing almost face to face with Carmella and A.J.; they get shot as well, either to remove the two closest witnesses or just out of spite for Tony. It's entirely possible that Tony's killer, if it is indeed the guy in the Member's Only Jacket who was eyeballing them in the diner, was specifically waiting for Tony's family to arrive before taking Tony out (notice that he enters immediately before A.J., and takes his stool at the bar as A.J. is walking to the booth; Member's Only guy doesn't know until A.J. sits down that he is Tony's son). Meadow is spared because she couldn't get her car parked, didn't take her symbolic final communion, and wasn't staring the assassin in the face at the moment he shot Tony. Granted, it's a rule of "Family" matters that families are untouchable; but this rule only holds if the assassin was affiliated with the mob, rather than one of the other hundreds of people whose lives have been ruined by, and hold a grudge against, Tony Soprano.

Alternatively, Meadow is killed moments after Tony.
If the killer is a mobster, Carmella and A.J. are left alone because families are not acceptable targets. However, at the moment that Tony is shot Meadow has just entered the diner; she's standing in the doorway. She is blocking the killer's escape route, and since she wasn't sitting at the booth with the rest of the family the killer doesn't know that Meadow is related to Tony. Poor girl.

Your cable actually did go out.
Tony ordered his men to tell the cable companies to pull the plug before the show finished.

Tony is alive. The ending is about paranoia, not death.
In the final scene Tony is constantly looking up to see whose coming through the door out of a sense of paranoia, seeing as its the final scene of the series we're in his shoes and fear that something terrible will happen. However, its really just to show us what its like to be Tony Soprano, living every moment in fear of whose going to come through that door.
 * Could it be that the blackout does not represent death, but the return of Tony's panic attacks?

The onion rings were poisoned
Mr. Chase explained his idea spot-on, but we cannot read minds. The only other direction would be the food. Members-Only-Guy would end up being a spectator. At this point who would refuse watching the pack of animals die? Another question would be why did the camera show chefs cooking in the kitchen? Why would the screenwriter put even that much emphasis there? Second being the onion rings, each one of them eats a ring in a communal fashion. Easy distraction can be an effect of a poison taking action. Don't think so? Just see how pale he and his son gets before Meadow comes in. The daughter comes in looking like she has something to hide or at least feel suspicious of. It's her watching daddy die.