Rhetoric.

Here’s a great site of primary sources for history teachers, composition instructors, and homeschoolers:  AmericanRhetoric.com.  The most important feature of the site is the Online Speech Bank:

THE ONLINE SPEECH BANKis an index to and growing database of 5000+ full text, audio and video (streaming) versions of public speeches, sermons, legal proceedings, lectures, debates, interviews, other recorded media events, and a declaration or two.

There’s also a "Top 100 speeches" of the 20th century.   And some famous speeches from movies, which (considering how much visibility they get compared to, say, C-SPAN) probably should be counted as influential speeches these days.

I found the site via a link from Betsy Newmark to Bill Cosby’s speech to the NAACP on the 50th anniversary of Brown vs. Board of Education.