The spam blacklist is a list of sites deemed spammy or otherwise objectionable – whose URLs are blocked from being added to wiki pages by mw:Extension:SpamBlacklist, currently in use on more than 2,000 MediaWiki installations, as listed here on WikiApiary. That extension is usually configured to pull its data from Meta-Wiki's spam blacklist page, found here. That blacklist contains a number of sites, such as Metapedia, that are blocked for political incorrectness rather than spamminess. Some sites, such as examiner.com, are blocked because they are considered unreliable sources that users should be prevented from adding to articles. Link shorteners such as TinyUrl are also blocked, since they could be used to work around the other spam blacklist restrictions.
It is questionable how effective this spam blacklist is at keeping spam off of third-party wikis. Some spam might be targeted only at Wikimedia Foundation wikis, or only at third-party wikis, which would make the list of little help to said third-party wikis. Also, some third-party wikis might prefer that users be allowed to cite sources that are not considered reliable on Wikipedia. Users may not always realize that, when a link is rejected as spammy, it does not necessarily mean that the individual wiki he is editing has specifically chosen to ban that URL.