The Internet Archive Wayback Machine allows users to “capture a web page as it appears now for use as a trusted citation in the future”. I’ve mentioned it before since it can be a very useful tool to find information on a webpage that has subsequently changed or been removed.
You are not restricted to using it to access materials that other people have archived. If, in the course of your research, you identify a useful web page, you may want to consider checking if the page has already been archived by the Wayback Machine and—if it is not already there—adding the page. Adding a page is very easy: simply enter the URL into the form and click on “save page now”.
Note that one drawback is that you can’t save a web page to the Internet Archive if the page is on a site that doesn’t allow crawlers.