Code comprehension. It's important when you jump into someone else's code.

Eclipse makes it easy, with F3, command-T, and shift-command-G.

Very quickly, you can jump through a maze of classes and interfaces, diving into details or seeing the higher hierarchy. I forgot how nice Eclipse can be. Particularly if you've got the viPlugin.

72 open files (and bless the JBossAS guys for including Eclipse projects and source jars).