RSyntaxTextArea 1.2.3 was just released on SourceForge. This is mostly a bugfix release, but it does have one interesting new feature – Mark Occurrences. After moving around, if the caret stays over an identifier or variable for a fixed length of time (around 1 second), all occurrences of that variable in the current source file will be marked. This lets you quickly get an idea of the usage of a variable or method in a class.
This handy feature has an API to enable or disable it to suit your tastes. See the setMarkOccurrences(boolean) and setMarkOccurrencesColor(Paint) methods in the Javadoc.