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.
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?
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!
You go over there and politely talk to them. You tell them how much you have respect for other peoples property and really get nervous that you may accidentally hit their vehicle when they park in the street. Then ask them if there is anyway they could park in another spot. Be sure to give them a compliment on their car. People love positive talk. That way then the ground is softened because what you are basically saying is that you are a horrible driver and can't fit your jalopy in your own driveway with out hitting their vehicle when they are...... Parked on a street...... Get my point? :0)