- You may want to see System administrators on Meta.
Developers are the people who write the MediaWiki software. They are said to have "commit access" (or to be "committers") if they have access to commit changes to the central Git repository by pushing changes for review in Gerrit where the latest copy of the software is stored.
This is in contrast to having shell or root access to Wikimedia Foundation servers, for instance, which would make them system administrators. Those with "root" or "shell" access might not really be developers, but rather system administrators of Wikimedia servers, where the code repository is stored. In many cases they rarely use their commit access, or if they do, only to maintain non-MediaWiki things in the repository.
Information about staff developers can be found at Foundation:Staff.
To see which developers are actually active, try http://www.ohloh.net/p/mediawiki/contributors, which gives nice stalkerish summaries of all sorts of fun statistics.
Who knows the most about, and maintains, MediaWiki core, key extensions? To be listed as a maintainer a person has to have +2 access to the relevant Git repository and regularly respond to bug reports and changeset review requests.
To avoid making this page too long, the table is kept at /Maintainers.
Developers formerly had an important role in the Wikipedia power structure, since they were the only ones able to create sysops or ban users. This has since changed, with the formation of user classes such as bureaucrat and Steward, and the technical capabilities given to these classes.
Some developers had shell access to the old California servers, but due to inactivity, weren't given shell access on the "new" Florida servers. Magnus Manske, Nick Reinking, Lee Daniel Crocker, Axel Boldt, Matthias Jordan, Neil Harris and Ed Poor are in this category. Note that Ed Poor was not involved with software development, but instead carried out what are now bureaucratic or sysop tasks, such as promoting sysops and locking accounts. Past MediaWiki software development activity level of developers may be found through CVS statistics.
From April 2006 to March 2012, Subversion was used; it came after CVS on Sourceforge, for which the following users had access: Angela Beesley (expired shell), E23 (snok) (expired shell), Tomasz Wegrzanowski (expired shell and root), Guillaume Blanchard, Axel Boldt, Rowan Collins, T.D. Corell, Lee Daniel Crocker, The Cunctator, Johan Dahlin, Peter Danenberg, Michael Dill, Andrew Dunbar, Emmanuel Engelhart, Andre Engels, Fantasy, Tom Gilder, Gabriel Wicke (expired shell and root), Jan Hidders, Anders Wegge Jakobsen, Matthias Jordan, Shane King, Thomas R. Koll, Niklas Laxström, Christian List, Wil Mahan, Magnus Manske (CVS admin), Zoran Obradovic, Julian Ostrow, Nicholas Pisarro, Jr., Evan Prodromou, Nick Reinking, Derek Ross, Valerio Santinelli, Chris Seaton, Alexander 'Ajvol' Sigachov, Chuck SMITH, Bogdan Stancescu, Marian Steinbach, Tarquin, Timwi (MediaZilla admin), Anders Törlind, Zhengzhu, and Rob Church.