Memory, time, fonts, system properties, packages loaded, PLAF's installed on host, system colors of host, contents of system clipboard, security settings, thread info, etc.
Programmers can hook it up to other programs to aid in rapid development.
Javert is a runtime Java class inspector that dynamically generates a mock source file based on what it finds out about the class you tell it to inspect. VirtualDice is like having your very own virtual bag of dice, of any size and quantity. This is of immense help to people who play board games and role-playing games, or who want to emulate them or implement as computer programs. TimeWatch is a Swing component for Java developers that always shows the current date and time. MemWatch is a simple Swing component that always displays the current used, free and max heap memory used by the currently running Java program.