Can I connect to a L2TP/IPSec VPN linux server using a Snow Leopard client? Do I need any special software?
What should I do to connect to a L2TP/IPSec VPN?
I would start with the built-in VPN software on the mac. You basically add a new network interface of the VPN kind.

Here's the quick tour:


Open System Preferences
select Network
add a new VPN interface to match your server settings




The Finder help menu has several articles worth reading - the top 5 hits show how to understand, set up, use as well as basic troubleshooting. Apple's support site has more in-depth information if you need to learn how to set up a VPN server (it's geared to the OS X server, but the concepts are the same on any server and once you get the terminology, the button location won't matter so much)