There is a couple of IPSec compatible VPN client:
- official cisco linux client
They all work well depending of the IPSec server. There is even a GUI for VPNC that integrate into Ubuntu network manager.
Official Cisco client is harder to install, require kernel headers, user-space binaries in 32 bits only. It can be tricky to install this one.
The problems starts with Xauth and Cisco devices when using non-Cisco hardware. I personnaly tried many times with occasional success.
Which distribution you use? Which IPsec server you connect to?
If you have ubuntu, you can give a try to vpn:
sudo apt-get install network-manager-vpnc vpnc
and set a new VPN connection using your configuration.