Is it illegal to park across the street from a driveway?

If you're parking in their property e.g.: Their own driveway, garage, or a designated area which is part of their property, then yes most likely it's trespassing when you park there. However, if you park at a street which is a public street, then the only restrictions that apply are those of the local government (the state, city, town .. etc). Someone doesn't have any legal support to ask someone to not park at a public parking because it's in front of there house. Out of courtesy, however, most people don't do it. But, again, it's totally legal and as long as it's public parking (street) you can park wherever you want following the rules of parking set by the government.