Cornell University Law School has posted an "annotated Constitution" with an extremely useful page dedicated to the First Amendment. It traces the First Amendment’s legal evolution and provides concise summaries of the critical court cases.
The First Amendment Center maintains an educational website with extensive information and resources pertaining to the religion clauses of the First Amendment. Several freedom of religion issues are summarized, such as school prayer and released time. There are also lesson plans and links to relevant congressional reports and summaries of pending cases.
Americans United for Separation of Church and State maintains a website that offers issue analyses, news reports, and case summaries. The organization is committed to a particular interpretation of the separation of church and state, but it provides a useful introduction to this interpretation.
The TeacherServe project of the National Humanities Center has a page devoted to religion in American history. Among the essays by leading scholars are two on the relationship between church and state in early America. The other essays will provide useful context for an exploration of the role of religion in American life.