To prevent conflicts in new namespaces added by extensions, please list your extension's custom namespaces here on the list of extension default namespaces.
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, extensions should reserve namespace numbers in blocks of 10, where no known namespaces are already added by MediaWiki core or any known extensions (see discussion).
Each block is intended for an extension to use sequentially; in particular, extensions that add more than one custom namespace should not reserve an additional block for each additional namespace unless the extension has exhausted its current block.
Namespace numbers between 3000 and 4999 should not be used; these numbers are reserved for wikis to define their 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.
The recommendations are good practices new extensions should adhere to avoid conflicts; for legacy reasons, old extensions that have already created conflicts and bad practices can't simply be changed but are listed here to help avoid even more conflicts.
This page in a nutshell:
MediaWiki core uses namespace numbers under 100. These must never be used by extensions or sites.
Avoid namespace numbers 100–199. These were historically used by both extensions and sites.
Custom namespaces on sites should use namespace numbers 3,000–4,999. These should not be used by extensions.
Extensions should use namespace numbers between 200 and 2,999, or between 5,000–9,999.
Avoid namespace numbers above 10,000. These are reserved for future use.
Two 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 the range 100–199 have historically been heavily used by both extensions and individual wikis for their custom namespaces. Because of this, it is generally recommended to avoid this range for new namespaces.
Wikimedia Foundation
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.
JSON schemas describing data models Note: Namespace name also used by LinkedWiki and (formerly) EntitySchema Note: Constant name also used by LinkedWiki
Additional JSON data. As of 2015 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
While this extension does not currently reserve a namespace in its code, it highly recommends creating a custom one for storing access control lists. Therefore, this has been reserved for sysadmins to choose a namespace easily should they not want the pressure to choose a random namespace.
Allows users to create maps that can later be included on other pages. Only enabled if $wgDataMapsNamespaceId is set to managed (default).
2901
Map_talk:
NS_MAP_TALK
ID 3000+: Reserved for sites' custom namespaces
The range of 3000–4999 is reserved for sites to define their custom namespaces without fear of conflict. No known extensions define namespaces in this range; new extensions should also avoid it.
The WSSpaces extension uses namespace constants >50000 to allow users to define custom namespaces.
Archived extensions and removed namespaces
Several extensions that added new namespaces have since been archived, and some namespaces that were added by an extension were later removed; these are listed here since these namespaces may appear in old database dumps from wikis which used these extensions, and some old wikis may continue to use the extensions.
It is very strongly recommended to avoid reusing these namespace numbers, as wikis that previously used the old extensions will have log entries etc. related to those pages; however, these numbers can be reserved by new extensions if desired.
Extensions are listed here ordered by the first namespace number they added.
JSON configuration (outdated) describing Zero site behavior for this carrier These namespaces are also defined in a test file for Extension:JsonConfig.
The documentation of this extension's namespaces on this page waschangedseveraltimes; it is not known if these changes were correcting wrong information, or if the namespace data was repeatedly changed in the extension itself.
Warning: 
Note: Namespace name also used by Wikibase