The Firefox CacheIt plugin lets you view cached versions of pages - it comes in pretty handy if the original source server is not accessible or if you want to view an older copy of the page.
CacheIt uses the Google Cache, Way-Back Machine, and the Coral (...and perhaps Dot Cache too?) caches to get cached copies of the page.
I used the Way-Back Machine to get a copy of David's Diary on Getting a Girlfriend, a popular Internet diary/blog from the late 90s. David's account no longer exists on the server so the diary was lost forever till Way-Back Machine traveled back through time and space and fetched a copy for me :-)