My new neighbors parks in street and makes it almost impossible for me to get in my driveway without hitting their car, esp. when there is snow.. Should I go to their house and ask them nicely or write them a letter. I don't have their phone number?

Best thing to do is go to their house and try talking to them. They may or may not know they are blocking you. If that doesn't work, then depending where you are, you can call your local Code Enforcement Dept. And they will come out and talk to them and cite them if they do it again. No letters, face to face is better and you'll know exactly what you are dealing with.

Go over. I always want my neighbors to talk with me first.

If these are your neighbors, you will be well-served to build a civil, if not outright friendly relationship with them. Go over and make nice, explain the situation politely, you'll likely get a more positive response than just sticking a note on their door. That could be interpreted as disrespectful or cowardly.

I agree with everyone here that you need to go and talk to your neighbours. The outcome is really going to depend on what kind of people they are. Working in car financing I've heard a lot of horror stories about rage on the road, but you live next to these people, so don't burn your bridges!