RI is the base architecture implementation for J2EE. Base on that, other vendors implement their own J2EE based frameworks, i.e. Sun offers RI for free which you can get from their website, that is RI, on the other side vendors like, Oracle, Borland, IONA, etc have their own implementation of the framework. in simple words, all it is, is the standard implementation of the framework and the rest are superset of RI.

When you download J2EE from sun site, It also contains J2EE RI.
Some of the tools provided in J2EE RI are trial or shareware.

Do you know JCP and it's JSRs?
A JSR is a part of the Java Community Process(JCP), which has a process in place to allow specification to be proposed, reviewed and finalized by a potentially larg number of community esperts . but before a JSR can be made public, it must also provide example code that not only validate the design, but also serves as a starting point for others who may wish to provide their own implementation . this example code, called Reference Implementation (RI), is a very important aspect of a JSR - many times, the actual implementation of a specification illustarates weakness in the design .

for example JSR 053, specifies both servlet and jsp and its RI is Apache Tomcat . RIs are always free for test purpose; but Integrating RI's to deploy a real applications is usually difficult for you and me !