Sharing files between Ubuntu and Windows or Mac systems can be done, but setting it up, until recently, wasn't quite simple. But Ubuntu's come a long way, and you can now get along without terminals, config file editing and intense hair-pulling.
A good guided walkthrough on the Ubuntu Forums promises none of the unfortunately common headaches of most Linux setups — there's only one terminal command, and it's actually optional. It's mostly a few right clicks, an IP address look-up, and simply asking two machines to find each other on the network. The guide is written for sharing between Ubuntu and Windows, but once you've set up the Ubuntu side, it's not too much more effort to load that Windows/Samba-shared folder on a Mac.
HOWTO: Easy, simple GUI setup of smb file server [Ubuntu Forums]