I sometimes need to debug JS in other browsers, and it would be really nice if they all had Firebug - failing that, what's the closest they have?
Fiddler is the one I use: http://www.fiddler2.com/fiddler2/
There is Charles: http://www.charlesproxy.com/ Also WireShark: http://www.wireshark.org/
How about Firebug Lite? It claims to work in IE, Opera and Safari by simply being a Javascript file.
Firebug can be used in other browser with it's javascript version : hhttp://getfirebug.com/firebuglite
Anyway, in Safari there is Web Inspector which is brilliant.
IE8 - Developer Tools (F12)
IE6,7,8 - Visual Studio/Microsoft Script Debugger/DynaTrace (for tracing JS, not debugging)
Chrome, Safari - Developer Tools.
For Safari and Google Chrome, the Developer Tools.
Just right-click anywhere and select "Inspect Element", or use the Ctrl + Shift + J shortcut:

- Firebug Lite - any up-to-date browser, but limited
- IE - number of options depending of IE version and do you have Visual Studio
- Opera - Dragonfly
- Webkit based browsers have inspector and debugger in menu
IMHO Firebug is the best tool but could be used only on Firefox
Opera has the excellent dragonfly developer console, which can profile your code (to find bottlenecks and redundent code) as well as step through breakpoints.