Words with Two Spellings

I am getting ready to go to see a musical tonight, the first of my much-anticipated season tickets to the Performance Series at the Washington Pavilion.  (That would be the place in town to see touring professional performances.)  I am still trying to decide if a subscription for one counts as season tickets or if it is simple a season ticket.  I got six tickets, but only one for each show…

Because I am going to see a show, I started thinking about the word theater.  This has two accepted spellings: theater and theatre.  One is more common in American English, but both are acceptable and your spell check usually won’t insist on one of them.

There are  other words I thought of with two possible spellings.  Specifically, I’m thinking of words that give relatively equal preference to both options; when there’s a clear regional or formal use preference (like color and colour), it’s usually best to stick with the more common or more formal one.

I thought of the following word choices:

Theater and Theatre

Gray and Grey (I personally prefer the E version.)

Dialog and Dialogue

Doughnut and Donut

Catsup and Ketchup (those are WAY different)

Can you think of any others with (nearly) equally preferred variations?