You can get ext 2 /3 drivers for MS Windows, how ever they are not so stable and often make Windows versions crash.......
How about using a NAS (network attached storage, please add to tooltip thing) for sharing your data between OSes? You get to share it between computers as a bonus.
Also, NTFS can now be writen to [with recent kernels]. I can not varify if it is stable enough
I bought a 200GB hard drive for the express purpose of test-driving multiple distros. My preferred distribution for the past two years or so has been BLAG (I switch back and forth between the current 60000 and 69999 Alpha), but I also periodically boot Fedora (currently 7) and the latest Linux Mint. So here, in order of most frequent use:
BLAG 69999 (based on Fedora 7)
BLAG 60000 (based on Fedora 6)
Linux Mint 3.0