Google Cache is a great solution if a web page is down. If you're visiting a site and it returns a 404 error message, you can either do a search on Google for that site (add the cache: operator, so your search query would be something like cache:www.google.com) or you can use this Firefox extension that provides a custom error page.
Google Cache shows a page like it was when Google last crawled it successfully, so it may be outdated. Google doesn't store images, scripts and other embedded objects, but it tries to retrieve them from their original location. If the page is down, most of the external resources won't be accessible and the page will load slowly.
In this case, it's a good idea to click on the "cached text" link to see only the text. You can also add &strip=1 at the end of the URL or install this Greasemonkey script that adds a link to the text version of the cached page in the search results page.
While reading the page, you may find other interesting links from the same site. Instead of repeating the same steps, you can install a Greasemonkey script that adds a new link to the Google Cache version for all the links from that page. This way, you can browse the web from Google Cache.
Oftentimes, pages load much faster from Google Cache and it's easier to spot your keywords as they are highlighted by Google. You'll also bypass access restrictions, so you can use it as a primitive proxy.