I've formatted my old 60GB USB external backup disk as a single ext3 partition. I figured it would be ideal to offload to it all my media files (videos, music, e-books and other documents) . But then I realized that I'll need to move stuff to and from this disk and my wife's Windows PC. So I started searching for a way to do it. I ended up trying out two ext2 file-system drivers for Windows: ext2ifs and ext2fsd.
Both are free and both seem to handle the basics well - read and write files from and to the disk. Both require a rather cumbersome mounting procedure where a special utility is used to assign a drive letter to the ext2/ext3 disk (but Linux users like me should not complain). Both fully support ext2 (which also covers ext3 partitions, but without journaling).
I tried ext2ifs first, and it worked nicely - I used it to move a 4GB DVD image from one computer to the other (perfectly legal, I assure you). It can't, however, handle UTF-8 encoded file names. This limitation is clearly stated in the ext2ifs FAQ, but I somehow missed it.
The ext2fsd changelog, on the other hand, indicates that it correctly handles UTF-8 and other file name encodings. And indeed it does. But unlike with ext2ifs, I could not "safely remove" the USB disk even after I unmounted the drive and exited the ext2 volume manager.
I'll stick with ext2fsd for the time being, mainly because it solves the UTF-8 problem, but also because it seems to be a rather active project with a growing set of features, while ext2ifs seems to have been frozen for over a year now.
[27 Jan. 2008] UPDATE: ext2fsd USB issues have been fixed since version 0.42, and even better - it can be configured to auto-mount USB disks. Sweet!