The Mozilla Javascript debugger, with a chrome-based user interface. It
has been included with Mozilla releases since 0.9.5, and is installable in
Netscape 6.2 or above using XPI packages. Some of its features include breakpoints, a console where debugging commands can be typed, a graphical call stack, profiling and the ability to view the code in a "pretty print" mode.
Although it is far from complete and features are still being added, as
with the rest of Mozilla, it is a very useful tool for web developers working with JavaScript on web pages,
as well as, of course, a valuable tool for the Mozilla developers themselves. It was improved dramatically between Mozilla 1.0 and 1.1, with a palette of nine views which can be docked or seperate, and the ability to write extra views using javascript.
To use it, either start Mozilla with the -venkman switch (necessary
to debug chrome code which would otherwise be loaded as part of the UI), or
click 'JavaScript Debugger' under the 'Tasks->Tools' menu.