Fordham University provides a list of links to online primary sources from the modern era.
This is the largest library in the world and contains documents that prove very useful for modern historians.
This is a free online textbook about the Vietnam War by Jerry DeGroot, a historian at the University of St Andrews in Scotland. This book was originally published back in 2000, but the author has now made it freely available online. It has a great bibliography and good academic discussion on the war.
An online academic database used by universities worldwide that is a superb resource for History students. To view full articles, you will require membership.
This searches internet sites for academic articles and books. It often only provides one or two pages, but it is often enough to find quotes and bibliographical details.