I was hoping to be able to at least read (though writing is always nice) ext3 filesystems on external drives from OS X, without having to start up a virtual machine. This page suggested I use OSXFuse and Fuse-ext2.
It turns out OSXFuse is the actually-maintained descendent of MacFUSE, but that it includes a “MacFUSE Compatibility Layer” (just a checkbox during install; no need for a separate download) that can handle the Fuse-ext2 module (which apparently hasn’t been updated in several years). Nonetheless, these things worked pretty seamlessly for me. I installed OSXFuse, then installed Fuse-ext2, then rebooted. The next external USB storage device I connected with an ext3 filesystem was successfully mounted read-only. Various other places on the web suggest that really slow writing should be possible, but I have not explored that option.