sending files...

[Duce]

WINS/SMB is by far the easiest and most elegant solution, and they are built into both OSX and Windows.  I've been using SMB/SAMBA with windows, osx, amiga os and linux off a central win 2003 file server for many years with excellent results.  The server part is entirely optional, just setup windows shares on the win box (es) and enable it on the mac side as Leffmann said.

[Duce]

If it's not all on the same network, FTP is likely your best bet, yeah.

Dropbox and other similar services may work as well if you have web access on all the machines, but if you are going to be doing it often FTP still is most likely the easiest.