When a site is offline, most of the time a recent version is still available on cache or mirror servers. This page allows simplified access to major cache or mirror services.
The Google Web Cache is the biggest cache provider. Most of recently offline pages can be found here.
The Archive.org cache stores multiple versions of webpages. If you're looking for an old version of a webpage, use the Archive.org cache.
Archive.is allows simple website mirrors which can be triggered by any user.