What are you looking for?

You could go with a PPTP server which uses Microsoft's proprietary VPN
and is available for Windows 98 and above without further
installation.

You could use IPSEC which is better but Microsoft mangled the standard
to have it look like PPTP, so it is difficult to use the normal way. 
This is really good for network-to-network VPNs.  IPCop has this
installed by default.  This will interoperate with Cisco without
hassle but Microsoft is different.

If you don't have any restrictions, you could install a less used open
format like OpenVPN.  You would have to install the client
applications for every computer, making sure that the client is
avialable for Windows, or Mac, or Linux, or Solaris.

I have used PPTP under a Windows only and IPSEC for network-to-network
connections.  I like IPSEC a lot but it did take a little getting used
to.  PPTP has some proprietary issues and is not in the Debian main
repository, however both Linux and Macintosh can run through PPTP just
fine.

IPSEC is integrated into the 2.6 kernel, although the tools to use it
are lagging a little behind (racoon-tool).  PPTP will have a module
that will have to be compiled and if you want to use domain
authentication, you will have to add winbind.

-- 
Jeff Rasmussen
GPG public key 0x9686C12F