The ending explained, very simply

Both earths were parallel until one saw the other, per news release. Therefore anything can change as of the night of the crash. Therefore the crash may not have happened on Earth 2 even if everything else leading up to it did. That is why she told John "they could be alive" when she left the ticket on his table.

Rhoda 2 showing up on Earth 1 proves the wife and kid were alive because Rhoda 2 had no one to feel guilty about and give her ticket away.

So many people got this wrong.

You're making a big mistake by taking the scene literally.
As it's the case in many films, the final scene is an opportunity for symbolism. The important thing isn't how Rhoda 2 came here or why, it's what it means on a symbolic level. The appearance of the second Earth was also symbolism. It's the place that's basically just like Earth but for the car accident or its outcome, the place where Rhoda imagines an alternate reality where she hadn't killed two people. That's why it's so important to her.

So, Rhoda is a broken person since the accident, like John.
First, she tries to help him directly out of guilt and shame. They have sex. He thinks he's falling in love with her. She confesses what she did, and he rejects her.
Then, she sacrifices her shot at living a new life by giving him the ticket, by explaining to him that his wife and kid(s) may be still alive on Earth 2. She realizes he needs it more than her. It gives John something missing from his life at that point: purpose and hope, as you can see on TV just before the mission starts, where he's again full of confidence and focus. She accepts her own fate: she'll stay in the world where she caused harm and death.
The gift still frees Rhoda from her depression, as you see her putting back furniture and flowers in her bedroom, which was previously bare walls.

So, it isn't very important to know how Rhoda 2 got there. This is not a cliffhanger for Another Earth 2 (Another Another Earth?????). We're not even supposed to spend much time guessing what happens to John in outer space. The important thing is that Rhoda the cleaning lady is face to face with Rhoda the MIT graduate in an alternate universe. She sees the side of her personality that she had put away due to the grief and the guilt. She may become whole again, or she may not be at ease with this part of herself, but she'll have to deal with it.

I'm surprised so few people on the board favor this view of the ending.
The idea that Rhoda 2 is a successful MIT graduate, who either just happens to be among the select few professionals to go on the mission, or wrote an equally moving essay about something else, seems far-fetched and doesn't support a symmetrical conclusion to the plot very well (literally or symbolically).
The interpretation described here aligns better with key points of the story and adds depth and a beautiful equilibrium.
Of course, the ending is as open as the rest of the film, but if I would settle on a version I would choose this one. Thanks to the OP for the insight.

I must be one DUMB MF'ER because I just don't get the ending.

1/ I agree she just can't give the ticket away to him. So the whole point mute.
2/ Even if she could give the ticket to John, does that mean John and family live happily ever after on Earth 2 - presuming John on Earth 2 was the only one killed.
3/ Rhonda on Earth 2 goes to Earth to meet herself as John go to his family on Earth 2. - What did they then did do - start a cat fight or become best friends.
4/ Man, I was totally confused and peeved when the credits rolled.
5/ Two hours wasted.
6/ OFF TOPIC: Gattaca(1997) was MUCH better.
7/ Gezz I even preferred the first few episodes of of EARTH 2 (TV Mini-series)

B!LL!

PS: I've read all the previous 'theories', but still don't get it, as many of them are conflicting anyway!

I wonder if there is any commentary on the DVD/Blu-Ray to explain a few things? - Namely the ending.

Plus I was hoping for a rocket ship/shuttle launch, that kinda would have made my two hours worth it, a LITTLE.

(8/ Earth 2 seemed to stay in the position relative to the moon. Rather odd that.)

9/ Rant over - sorry about all that !!!!!!!!!!!!!!!!!!!!!!!!!

Why is everyone talking like we know what happened on Earth 2? All we know is that Rhoda 2 didn't give her ticket away. Anything else is pure speculation... and that's kind of the point.

Only the closed mind is certain.

The ending spoiled the whole film. It was lazy. The concept was good and running along fine and then the writer ran out of ideas. What sunk it for me was that surely the arrival of Earth 2 travelers on Earth would have been the greatest event ever to have occurred in the history of mankind and as such every second of it would have been covered by the world's media. How on earth could Rhoda have been unaware that Rhoda 2 had arrived on Earth? As for the idea that these travelers could then just wander around on their own without attracting any interest from anybody else just hammers a nail into the film's coffin. It could have been a great film but it ended up sucking dog balls.

There is no actual answer to what had really happened and, consequently, why or how Rhoda 2 came to earth. The important thing is Rhoda, after everything she has gone through and done, finds redemption and at the end of the movie she 'finds herself' on a methaphorical level. This seeing and finding oneself situation is a referance to that old man, who blinded himself because he didn't wanna see himself anymore and bleached his ears, leaving himself deaf. When Rhoda goes to his room, he says "You are probably wondering why. No. You know why." And Rhoda writes "Forgive" in his palm, making him cry.

Was she helping him, or herself? Was it the earth that she was going to run away, or herself? Maybe, just like that old man, she didn't wanna see herself anymore.

So, unlike that oldman who couldn't forgive and didn't wanna see himself, she was able to 'forgive' herself and that is why Rhoda 2 was in Earth 1, in front of Rhoda 1, so that she could see herself once again. She can look at herself now that she has found forgiveness.

Having said that, this means that none of your guesses are true, but at the same time any of them can explain the ending as long as they make sense.

Bobbykanae

Very insightful...I think you are correct. Thanks for the great post!

BUT as for your "sidenote"...I have to disagree...John was actually happy for her that she won the contest(although he wanted he to stay on Earth 2 due his growing feelings for her). He only got angry after she told him her story of how she caused the crash. I don't believe that he had entered the contest....my opinion...