Spectator is a XUL application, which should make it cross platform. I have tinkered a bit with XUL before, but not a full application. [...] So what can spectator do? Mind that this is a first version and I really just meant it as a proof of concept. I think I got a bit further than that, but it probably still has a few bugs. Still, with the current version, you can step through a program, set breakpoints and inspect the stack. Really all you would expect from a debugger.
You can grab the latest version from the subversion repository on the Google Code website.