To be able to read ext3 formatted disks on Mac OSX Lion (10.7) you need to install a current FUSE implementation (macFUSE is discontinued, but there are several newer projects):
Then, you need to install the filesystem itself. The
fuse-ext2 is just right (and supports
ext3 as well):
After installing both packages you should be able to plug-in a
ext3 formatted linux disk and it should show up in your finder (read-only).