Wikipedia is the largest wiki on the planet, with several million articles available in English and several million more in dozens of available languages.
For once, a site that recognizes the importance of third-party backups! They have a main downloads page from which you can get XML dumps from individual wikis (Wikimedia Foundation hosts more than 700 wikis: Wikipedias, Wiktionaries, Wikinews, Wikisources, Wikibooks, Wikiquotes, Wikiversities, Wikispecies, Wikimedia Commons).
You can download all the articles of the English Wikipedia (with complete edit history) in an unique file (compressed in 7zip format) from here (ATTENTION: 31 GB! Unpacked it expands up to 5.2 TB. Direct link: pages-meta-history.xml.7z).
Some Wikipedia dumps in the Internet Archive.
There is no current public backup for images uploaded to Wikimedia Commons which has about 10 million images and other media files uploaded on it's services.
Stable, but they seriously use a lot of tactics to get donations.
- http://download.wikimedia.org/archive/ some incomplete old dumps, English Wikipedia mainly
- old wikipedia backups discovered http://noc.wikimedia.org/~tstarling/wikipedia-logs-2001-08-17.7z