Cross-platform file system

As Breakthrough said, use NTFS. In both Mac OS and Linux, you can use NTFS-3G to enable read/write to an NTFS partition.

On OS X, NTFS-3G can also be installed through Homebrew for free with brew install ntfs-3g. You also need Fuse for OS X, but that's it.

These projects are free, open-source and mature. I've used this setup on a Mac and I've had no problems accessing data from an NTFS partition.

UDF is a candidate. It works out-of-the-box on linux >= 2.6.31, Windows >= Vista, MacOS >= 9 and on many BSDs.

Note: UDF comes in different versions, which are not equally supported on all platforms, see Wikipedia - Compatibility.

Related question: Using UDF on a USB flash drive

The simple answer is- no. There is no lowest common denominator across these OSes aside from FAT32.

By lowest common denominator, I mean built-in filesystems. For add-ons, you're on your own.