LAN is good.
If you have a hub or router, or even if you can attach the two directly by Ethernet cable, you can create a shared folder on the PC and log onto it from the Mac OSX using Command-K, smb://[yourpc's IP address] -- you can also create a Personal File Sharing shared folder on tha Mac and log on from Windows - theres probabaly several other ways that other MacRumorites will be quick to correct me on.
One thing you'll have to watch a bit between Mac and PC is the length of file names, and "illegal" characters in the file names.
You can also burn a CD - burn it as a Disk at Once CD, not a Packet-writing CD.
USB keychain flash drives also work for smaller amounts of data.
You can share a drive between Mac and PC, but it can only be physically connected to one machine at a time. You can network a shared folder on it to the other machine, however.
An external firewire drive could be moved from one machine to the other, if the PC has firewire. You would have to format it with FAT32 for this to work.
Or if you are buying something, you could look at a Network Attached Storage device, which is basically a hard drive with a tiny Linux server and ethernet port included, that plugs into your network. This would be a fileserver that both machines could use and back up to.
Anyway, you will have lots of ways to interoperate.
Thanks
Trevor
CanadaRAM.com
Edit: Lee types too fast
