Solution 1:

MacFUSE OSXFuse plus fuse-ext2

Note: fuse-ext2 was previously hosted at http://sourceforge.net/projects/fuse-ext2/.

Solution 2:

I find the easiest and most reliable way to access linux ext3 partitions on my mac is to run Ubuntu in a vmware virtual machine on the mac. Once running you can connect Ubuntu to the ext3 drive via usb, and this way you can get complete reliability reading and writing to ext3 file systems. It's fast because the disk is directly connected to the mac, unlike on a network. And you can use it to copy files very quickly to/from the mac host by using shared directories or any number of other ways.

Solution 3:

You could also install either e2fsprogs or ext2fuse with macports.

Don't think any mount commands are included with port's e2fsprogs. (Though there's fsck and the like.) But installing ext2fuse does do the trick!

Solution 4:

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

BTW, recently I suffered from a Mac OS crash apparently due to fuse-ext2
https://sourceforge.net/projects/fuse-ext2/forums/forum/787602/topic/6549364