So far I have seen many of the same features mentioned with IntelliJ that either come with Eclipse or can be added in for FREE. Can anyone provide me with the chief advantage I would gain after spending $500 on IntelliJ?
Also, many commercial products include plug-ins to Eclipse which enable them work with their products. Which large software vendors are partnering with IntelliJ providing add-ons which enable work with their products?
The main advantage is simple: productivity and pleasure. Of course, if you compare Eclipse and all the plugins available for it with IntelliJ IDEA by counting checkmarks on the feature matrix, Eclipse will be far ahead. However, you surely know that the quality of a product consists of many things besides such checkmarks. You'll often find that the user experience and the UI quality of the free plugins available for Eclipse, and sometimes of Eclipse itself, is far from perfect. On the other hand, IntelliJ IDEA strives to provide a high-quality integrated user experience for all the supported functionality.
Which particular software vendors are you interested in? In many cases, the integration is not provided by the software vendors but rather by JetBrains itself or by members of the JetBrains plugin development community.