If you have OSX 10.6.5 or later, and Windows Vista or later, you can format the drive using the exFAT file system and both operating systems will be able to read the drive. However, I think you have to format it with the Windows system for it to be readable in both OS's.
The caveat is that most people say not to format hard disk drives with exFAT, and that you should only use it for flash drives. I did it with my Western Digital 2TB drive and so far it's been working well.
As GGJstudios suggests, you won't be able to use it with Time Machine.
**CAUTION: This will delete everything on your drive (or the partition that you're formatting).**
Plug the drive in the Windows system, right click it in Explorer, and click "Format..." Then choose the exFAT file system and use the default allocation table size unless you know what is best for your system (Google it), and click the "Quick Format" box. Click Format and you're good to go.