The most common is the good old airship in the modern day world. I'd like to hear about some others. So see this as a writing or thinking exercise, or just a bit of fun.
The setting is 2006. You land in a version of the world where things are just a little different. The cities/people are more or less the same but there are other changes. Let's not go for airships (unless it compliments you ideas), that's the classic, and no 'nazi's won the war', either. We're just after changes that make you do a double-take as you realise this isn't your Earth.
king_espresso came up with two great ones - Elvis Concert Tour posters and the Twin Towers intact. No need to try and match other people's ideas, just write what you want.
Here's one to start you off. The microchip revolution never happened. No mobile phones or computer shops. Lots of phone booths and the like. Televisions are still big as are cameras. VCR's are huge beasts.
So, what it's like in your parallel universe?