Synchronicity offers six options to support the Internet user in (re-)discovering missing web pages.
Install it and you will find a little shrimp icon in the right corner of your status bar. Whenever a 404 "Page not Found" error occurs Synchronicity triggers and its icon turns red. Now you can click on the icon once in order to load the panel. You can also activate Synchronicity for all other (not missing) pages by double clicking the icon.
With the help of the Memento framework Synchronicity offers archived versions of web pages. It provides two visualizations of all available previous copies of the pages in dependence of their archival date and repository.
The TimeGraph is a static image providing an overview of all available copies per year. Three drop down boxes enable the user to pick one particular copy by date and have it display in the main browser window.
A Simile TimeLine offers a "zoomable" way to browse through all copies. Click on the icon of the archive the copy is available from to obtain metadata about the version. All copies can be filtered by archive.
Based on these previous versions Synchronicity provides two content based methods:
1) the title of the page
2) its keywords (lexical signature)
that can be used for search engine queries to rediscover the missing page at its new location or obtain a good enough replacement page. Searches in Google, Yahoo! and Bing are offered.
Synchronicity further tries to obtain tags from Delicious that again can be used as query strings.
It also extracts extended keywords based on content of the link neighborhood (inlink pages) of the missing page that can be queried.
The user can lastly edit the URL of the page that caused the 404 error. Maybe shortening it can help find what you are looking for.
Synchronicity offers an Expert Interface for the experienced user. Here you for example can toggle between Memento proxies. You can also pick one particular copy to extract the title and keywords from. The add-on further offers all kinds of parameters to be modified for titles, keywords, tags and extended keywords.