Working with CSS - here's some handy CSS packages that enable easy drop-down menus that behave nicely with non-css browsers:
http://htmldog.com/articles/suckerfish/ <--- good place to start
Working with Javascript:
- Firebug (the obvious way to start):
- Internet Explorer work-arounds (working around the lack of the window.console):
http://ryan.ifupdown.com/2008/08/29/stop-firebug-console-log-errors/ avoids the errors by installing a window.console with no-op methods ( http://ryan.ifupdown.com/wp-content/uploads/2008/08/noconsole.js )
http://icant.co.uk/sandbox/fauxconsole/ shows you the log messages (more useful if you need to do debugging from within internet explorer)