IntelliJ IDEA provides JUnit integration out of the box, but it does not mean that you cannot use other test frameworks. TestNG-J is a plugin that provides integration with the TestNG framework in a very similar way to the native IntelliJ IDEA integration with JUnit.
TestNG-J adds its own type of run/debug configuration that allows for running tests. It also displays testing results and highlights failed and passed tests. To learn more about plugin features and configuration, take a look at the plugin overivew.
The plugin is free and can be downloaded and installed using IntelliJ IDEA Plugin Manager. The plugin is available for both IntelliJ IDEA 5.x and EAP versions of IntelliJ IDEA 6.0.