For those occasions when I've needed to work with Linux ext filesystems on my Mac, I've had good luck with Paragon Software's extFS for Mac product. It retails for ~$40, but I've seen it included with bundles for significantly less every now and then.
Another option, of course, would be to keep a Linux system handy, especially if there is a need to access less frequently used filesystems, like ZFS, ReiserFS, or XFS. I'd expect a Linux virtual machine on a Mac would work, too, as long as the vm controls the USB device or other device containing the filesystem of interest, but I haven't tried it. One caveat is that not all Linux distributions enable support for all Linux filesystem types by default, so additional configuration may be required in some cases.