During my internship at the American Registry for Internet Numbers I had to build a VPN server based on IPsec that supports every 'common' client out there. So I built a VPN Server that works for Windows XP, Mac OS X, Linux, FreeBSD, OpenBSD and Windows CE. Probably it supports even more clients, but I didn't test it with more. You can find my detailed documentation about all this right here. Choose your format:
In 2008 I re-started this project to mainly support Mac OS X Clients, see the results.