Interoperating: Mac OS X

Since Mac OS X is based on FreeBSD it should interoperate just like FreeBSD.
To use Appletalk over IPsec tunnels, run it over TCP/IP or use Open Door Networks' Shareway IP tool, described here.
James Carter's instructions (X.509, NAT-T)

MacOS X 10.3 ("Panther") and 10.4 ("Tiger") support L2TP through the built-in "Internet Connect" application. See Jacco de Leeuw's article, Using a Linux L2TP/IPsec VPN server with MacOS X. Tiger supports both certificates and PSKs, Panther only supports PSKs.

MacOS X's 10.2+ IPsec implementation can be manually configured so that L2TP is not required. This requires some Unix command-line experiece: text configuration files will have to be edited. But there are also graphical front-ends available which make this task easier for less experienced users:

IPSecuritas adds a graphical front-end to the built-in IPSec core, "allowing you to setup secure communications within minutes". It is freeware and known to work with Openswan.
VaporSec is a freeware GUI. It is reported to work.
VPN Tracker from equinux is guaranteed to work with FreeS/WAN. This is a commercial product. According to equinux, "it should work" with Openswan as well.

If you are using Mac OS X 10.4.0 ("Tiger") with one of these third-party VPN clients you will need to install update 10.4.1 or higher due to bugs in 10.4.0.

If you prefer to use XAUTH authentication you can use the Cisco VPN client which is available for the Mac. Reportedly, Tiger already contains support for XAUTH. To use XAUTH on the client (either the Cisco VPN client or Tiger) you will need XAUTH server support in Openswan. Interoperability status is unknown.