Firebug Lite is an acceptable cross-browser solution. In OpenLayers, we include a patched version that works a bit better in IE. While nothing like a debugger, Firebug Lite provides simple command-line access outside of Firefox.
My new favorite for work outside of Firefox is Jash. Especially handy is the little bookmarklet (drag to your bookmarks). Open up a page with a map, click your Jash bookmarklet, and start exploring. This is particularly nice for remote debugging.
>> map [object Object]
Start typing map.getZ and hit tab for completion, see
Hit Ctrl-X and mouse over the elements to see the DOM hierarchy. Enter jash.help() for more.