Teleport is a free software for Mac OSX:
http://abyssoft.com/software/teleport/
It isn't compatible with Windows though. It works only between Macs.
Multiplicity for Windows:
http://www.stardock.com/products/multiplicity/
From the Wikipedia article:
Multiplicity comes in two versions,
the standard Multiplicity and
Multiplicity Pro. The standard version
has all the above features, including
the ability to copy and paste images
and text between computers. The Pro
version also allows the user to
control up to six client computers as
well as the ability to copy and paste
files, folders, and other data between
machines.
(...) A
Mac OS X client is in development, due
in part to the popularity of the Mac
mini. A Linux client is reportedly
planned, but has not yet been
developed.