fuse-ext2

fuse-ext2 works but is very slow (I get some 7 MB/s reading and 1 MB/s writing via USB 2.0). If you have the choice, better use Apple's HFS+ on the external drive, which is much faster (I get some 30 MB/s both reading and writing, both on Mac and Linux). HFS+ is supported by Linux mostly out-of-the-box. To get write access even to a journaled partition on Ubuntu, install support via 'sudo apt-get install hfsprogs' and when the drive is mounted (e.g., automatically), re-mount it writable using 'sudo mount -o remount,force,rw /mount/point'