FAT32 for native read/write support in both OSes, however, FAT32 has a size limit on files; 4GB.

NTFS would be better, but you need to install either MacFuse+NTFS-3G or Paragon NTFS in order to write to an NTFS partition, as Leopard only has native NTFS read support.

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Mac OS X Leopard 10.5.8 (9L30) | Chameleon 2.0 RC3 | Intel Pentium 4 3Ghz 800Mhz FSB HyperThread 1MB L2 cache SSE3 1MB L2 cache works, cosmetic display of 512KB L2 cache | Asus P4V8X-MX VIA Chipset, VIA-VT8237 Southbridge | AC97 VIA8237 | Dual 1GB 333Mhz DDR SDRAM | nVidia PNY GeForce 7600GS 512MB 8x AGP DVI/TV-Out/VGA [NVinject 0.2.1; QE/CI/QuartzGL/Rotation] | Darwin Kernel Version 9.7.0: Sun Jun 14 20:48:28 IST 2009; Voodoo 2.0 Intel alpha3 :xnu-1228.12.14/BUILD/obj/RELEASE_I386 i386