I am working on a web application that needs to support the widest possible audience. I have errors popping up in Konqueror that don't occur in major browsers.
So I'm curious, are there any JavaScript (or CSS for that matter) debugging tools for any low-market share browsers, or is there some platform agnostic way to ensure proper execution in almost all environments.
If you are going to build for older browsers and operating systems, than my suggestion is not to duplicate the experience, but to use Progressive Enhancement (PE). In my experience, this is a better opportunity for providing a satisfactory user experience across the board. That I know of, there just isn't any other way to guarantee a decent end-product for almost all environments. If I use PE, then I know that I can test against A-Grade browsers, and still provide functionality for the small percentage that are unlucky enough to be surfing with IE 5.5 or Netscape 6 -- that is, without having to test each and every one of those.
http://www.alistapart.com/articles/understandingprogressiveenhancement
If you want some additional testing capabilities beyond that, then for IE I suggest getting your hands on Multiple IE by TredoSoft. For other browsers you can always try to install them from Evolt's Browser Archive.
There's no platform agnostic tool. Each browser has its own bugs and peculiarities which would make such a thing impossible to write.
In general, if you have the resources then you can test on as many browsers as you can find and either use the built-in debugger (if it has one) or resort to the old alert() style.
What's more important is that your site should degrade gracefully if a javascript error does occur, or the user has javascript disabled.
PS you say "almost all" environments - IE, FF and Opera have that pretty much covered :)
Konqueror hasn't got a javascript debugger, AFAIK.
Try to have a look at the debug output (lines that start with "JS:"), or go the good old "alert()" way.