Ctrl+Q. Do it often, do it everywhere:

It brings up the Javadoc, nicely formatted -- and with clickable links.

Javadoc not available? So just download the jar with the relevant Javadoc, and add it to one of your libraries.

Here's an example. Assume you have struts.jar in a library (a library is just a named collection of jars, which can contain source code, binary code, or javadocs).

• File->Settings->Modules
• Click on your module in the left pane.
• Click Libraries (Classpath).
  
• Click on the library in the left pane which contains struts.jar.
• Click Edit.
• Click either Attach Javadoc... (if you've downloaded it (you need to unzip it)), or Specify Javadoc URL.. if you just have an url of it on the Web.

Voila: after doing this, Ctrl-Q will bring up the javadoc of any method, class, var, what-have-you.