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 8 million images 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