Not sure why it's so hard to find this data all put together so here it is. Direct download links for the Spring JARs and the spring xml file header configuration for each version.
Spring 2.0 - October 2006
Direct Download Link: Spring Framework 2.0.8 (latest Spring 2.0.x release)
Spring 2.5 - November 2007
Direct Download Link: spring-framework-2.5.6.SEC03.zip (latest Spring 2.5.x maintenance release)
Note:The only difference between the syntax for Spring 2.0 and 2.5 seems to be the addition of the context schema in 2.5.
Also as of Spring 2.5, Spring provides support for configuring the MVC framework components using annotations.
Spring 3.0 - December 2009
Direct Download Link: spring-framework-3.1.3.RELEASE.zip (current production release)
Note:The spring.jar that contained almost the entire framework is no longer provided in Spring 3.0 so you must include each jar as needed separately.
New Features in Spring 3.0
Spring 3.0 also introduced a few new features such as additional annotations for MVC, server-side support for building RESTful applications, the Spring Expression Language and integration of some core features from the JavaConfig project.
Useful LinksSpring 2.0.x Reference
Spring 2.0.x Schema Configuration
Spring 2.5.x Reference
Spring 2.5.x Schema Configuration
Spring 3.0.x Reference
Spring 3.0.x Schema Configuration
New in Spring 3.0
Spring Main Download Page