We branch and tag extensions along with versions of MediaWiki, so the code found at http://svn.wikimedia.org/svnroot/mediawiki/tags/REL1_15_5/extensions/ is supposed to work with MW 1.15.5. However, this assumes that the trunk version of each extension worked with the trunk version of MW at the time of the branch point, which is not always the case and is the responsibility of the extension maintainer. Versioned releases can also be downloaded through ExtensionDistributor at mediawiki.org, although ED breaks all the time and we've been getting lots of questions in #mediawiki from people whose wiki broke because they installed the trunk version of ParserFunctions (downloaded through ED) with MediaWiki 1.15.
Though branches, tags, and the extension distributor exist, in reality none of them come even close to solving the problem.
The issue is, none of these things allow an extension author to properly match a version of an extension to a version of MediaWiki. The solution I take is to always have my extension useable in the trunk version, and backwards compatible to as many versions of MediaWiki as possible.
This is a problem we need to solve. This is one of the things that makes upgrading MediaWiki a crap shoot.
V/r,
Ryan Lane