Hard drive formats which can be written to and booted off of

Macs can't write to NTFS formatted partitions without:

http://code.google.com/p/macfuse/

And they can't boot from them either regardless of what software is used.

Networking your Mac to the PC with such a hard drive attached is another way to get the Mac to be able to read and write to it:

http://support.apple.com/kb/HT2959

PCs can't read or write Mac HFS+ formatted partitions without:
http://www.mediafour.com/ 's Macdrive.

Macs can write to FAT32 partitions (which are limited to 32 GB), but can't boot off them.

PowerPC Macs can boot off HFS+ with formatting with Apple Partition Map partitioning in the Partition section's Option button of Disk Utility. And that is assuming the operating system on that drive is supported by that Mac as this article describes as well as the spec pages below it:

http://support.apple.com/kb/HT2191
http://www.info.apple.com/support/applespec.html

Intel Macs can boot off HFS+ with formatting with GUID
partitioning in the Partition section's Option button of Disk Utility. This article tells you the minimum operating system that the Intel Mac supports:

http://support.apple.com/kb/HT1159

Note you can't boot off a PowerPC operating system on an Intel Mac, or Intel Mac formatted operating system on a PowerPC Mac.

You can make a dual-boot PowerPC Intel drive with the instructions on Mac OS X Hints:

http://www.macosxhints.com/article.php?story=2006061610374449

Additional info can be found on this tip.