There have been so many of these that there is no way to put a significant percentage of them on here. For example:
- Lemon v. Kurtzman. This was a case having to do with the Establishment Clause. It set up a test to determine what government actions that seem to favor religion are actually acceptable.
- Miller v. California. This one also set up a three part test. This one determines what things are and are not obscene.
- Johnson v. Texas. This one struck down a law against flag burning.
- Near v. Minnesota. This one said that governments cannot, in most circumstances, exercise "prior restraint" to prevent a newspaper from publishing something.
Again, there are so many more that have been very significant, partly because there are so many aspects of First Amendment law.
Engle v. Vitale- the decision that required bible and prayer study in school is unconstitutional.
A case that has since been diminished is Schenck v. US
Those are the only two I can come up with off the top of my head, but I hope they help