To prevent conflicts in new namespaces added by extensions, please list your extension's custom namespaces here. If you define a constant for a custom namespace, list it here too.
Note that these are only default namespaces. All extensions defining new namespaces should provide the installer with a method of configuring the extension to use a different range of namespaces for its custom namespaces.
In general, numbers for new namespaces should be reserved in blocks of 10, where no known namespaces are already added by MediaWiki core or any known extensions (see discussion). Namespace numbers between 3000 and 4999 should not be used; these numbers are reserved for wikis to define their own custom namespaces. Furthermore, care should be taken when selecting numbers under 1000, since this range has historically been quite heavily used by extensions and sites alike (numbers under 200 should probably be avoided altogether).
Naturally some namespaces listed on this page will overlap, and violate the recommendations listed here. The recommendations are good practices new extensions should try to adhere to to avoid conflicts; for legacy reasons, old extensions that have already created conflicts and bad practice can't simply be changed but are listed here to help avoid even more conflicts.
In a nutshell:
Namespace numbers under 100 are reserved for MediaWiki core and should never be used by extensions or sites.
Namespace numbers 100-199 should generally be avoided, because of heavy historical use by extensions and sites alike.
Namespace numbers 3000-4999 are reserved for sites to define custom namespaces, and should not be used by extensions.
Namespace numbers above 5000 should generally be avoided unless there's a good reason to use them.
Note:2 namespaces have negative indexes and have special purposes. You cannot create or delete pages in these namespaces, and there are no corresponding discussion namespaces.
The namespaces in 100-199 are reserved for site-specific namespaces, and should not be used by extensions. Many extensions are already using this range, though, so be careful. If you're writing a new extension you should never hardcode any namespaces inside the 100-199 range.
Many Wikimedia Foundation wikis have site-specific namespaces in this range. Including the Portal namespace (100) on most Wikipedia projects, Creator namespace (100) on Wikimedia Commons, the Extension namespace (102) on mediawiki.org, the Subject namespace (112) on English Wikibooks, and many others.
300 - 399. Wikia has been seen using namespaces in this range for its custom extensions and configurations. To avoid conflicts in case Wikia extensions are ported to work outside of Wikia it would be best to avoid defining new extensions that use this range. Wikia alters the namespaces for some extensions used on its own wikis. Please also take these into account while coming up with new namespace numbers to use in new extensions.
JSON schemas describing data models Note: Namespace name also used by LinkedWiki and (formerly) WikibaseSchema Note: Constant name also used by LinkedWiki
JSON configuration (outdated) describing Zero site behavior for this carrier These namespaces are also defined in a test file for Extension:JsonConfig.
Additional JSON data. As of 2015-02[update]}} you can store arbitrary valid JSON in the subnamespace Data:Json, e.g. Data:Json:MyDataset. This is a developing convention on Wikimedia projects that use JsonConfig and Graph. Note: Namespace and constant name also used by LinkedWiki
Some sites try to make use of the 500-599 range for site namespaces.[citation needed] To avoid conflicting with those sites it would be a good idea to avoid this range while creating new extensions.
The 1000+ range starts to get fairly high and is a range some sites may attempt to use to avoid conflicts with extensions. So be wary of defining new namespaces within this range, especially near 1000.
Note:Translate extension uses these namespaces by default, but they can be overridden. FYI translatewiki.net uses namespaces from 1200 onward to hold product translations.
Note: Namespace and constant name also used by BlogPage
1503
Blog_talk:
NS_BLOG_TALK
Note: Namespace and constant name also used by BlogPage
1504
Book:
NS_BOOK
1505
Book_talk:
NS_BOOK_TALK
Note:The BlueSpiceExtensions bundle contains only two extensions with a namespace at the moment. This may change in the near future. Therefore the Hallo Welt! team would appreciate it if the range of 1500-1599 could be reserved for this package.
So far, no known extensions have defined default namespaces in the 3000+ range. For now, it would be best to avoid using this range for extensions, so that sites can use this range to define their custom namespaces without fear of conflict.
In accordance with this, the BlueSpice Namespace Manager uses the range above 3000 to create custom namespaces.
Warning: 
Note:2 namespaces have negative indexes and have special purposes. You cannot create or delete pages in these namespaces, and there are no corresponding discussion namespaces.