Taverna 2.x is no longer actively developed. Development continues for Taverna 3.x as part of Apache Taverna.
The historical source code for Taverna 2.5 is available as a series of Maven modules from the Taverna GitHub repositories. Tag names for version 2.5 is typically $module-1.5 or $module-2.5. See the Maven version numbers for an (outdated) overview of the version number per module.
For comparisons, the Apache Taverna source code also include historical tags for the 2.x modules. For instance, the old/core-1.5 tag of incubator-taverna-engine was imported from the tag core-1.5 on taverna-engine-core – which corresponds to the core-1.5 branch in the former SVN structure. (Unfortunately no tags were made for the 2.5 release in SVN).
Note that the branch maintenance on the taverna GitHub repositories represent any unreleased 2.x development (if any) – which is snapshot in Apache Taverna source code with tags like old/core-maintenance-2010-09-06.
For Taverna 2.4 and earlier you can also browse a copy of the former SVN repository which was previously hosted at http://taverna.googlecode.com/svn/ – note that this corresponds 1:1 to the SVN structure with nested tags/branches/trunk structure.
Some additional Taverna tools are in the myGrid GitHub repositories.
You may also download the source code for the older releases.