You can absolutely mount ext2/3/4 drives within OS X using FUSE (MacFUSE). While MacFUSE is no longer in development, its latest version should allow you to mount and use a number of unsupported formats, including ext3.
1. Install MacFUSE: http://code.google.com/p/macfuse/
2. Install the ext2 FUSE module: http://sourceforge.net/projects/fuse-ext2/
3. Use Disk Utility to get the device name of the ext3 drive by enabling the debug menu and showing all partitions (see here: http://reviews.cnet.com/8301-13727_7-20081463-263/manage-all-partitions-with-dis k-utility-in-os-x/). Then select the ext3 partition and get info on it to get its device name (e.g., disk2s2).
4. Create a mount point by making a directory in your /Volumes folder or anywhere on your system.
5. Mount the ext3 drive with the following command in the Terminal (replace "disk2s2" with whatever your device name is, and likewise for the specified mount point):
fuse-ext2 /dev/disk2s2 /Volumes/mountpoint
To add write support, supply the "-o force" flag as follows:
fuse-ext2 -o force /dev/disk2s2 /Volumes/mountpoint
(No guarantee on good r/w performance, but it is an option)