Building on Ken's answer: I used fuse4x and fuse-ext2 successfully, and I recommend fuse-ext2 over ext4fuse. fuse-ext2 provides write access while ext4fuse provides only read access. Note that despite its name, fuse-ext2 supports EXT2/EXT3/EXT4.
ext4fuse was a hassle for me, because it requires manual compilation and has no support for fuse4x options that would allow me to set access control. fuse-ext2 provides downloadable packages, and the 0.0.7 version worked just fine. I copied a few large ISOs over without any problem.
Edit: It should be mentioned that as of 12/22, the fuse-ext2 Github page still lists unstable write support as a bug and recommends not using it "unless you have nothing to lose."