L2tp VPN connection plugin?

Regarding Werner Jaeger's PPA: l2tp-ipsec-vpn seems to be in normal quantal/universe repository now, so you don't need adding any special PPA for it...

Open up a command prompt and type the following to install the client:

sudo apt-add-repository ppa:werner-jaeger/ppa-werner-vpn
sudo apt-get update
sudo apt-get install l2tp-ipsec-vpn
reboot

Check out this guide for a full tutorial for configuring connection options after installation. This is the easiest way that I have found for setting up an L2TP client on Ubuntu.

Use https://launchpad.net/~seriy-pr/+archive/network-manager-l2tp

sudo apt-add-repository ppa:seriy-pr/network-manager-l2tp
sudo apt-get update
sudo apt-get install network-manager-l2tp-gnome

sudo service xl2tpd stop
sudo update-rc.d xl2tpd disable

NEW for 16.04:

sudo apt install intltool libtool network-manager-dev libnm-util-dev libnm-glib-dev libnm-glib-vpn-dev libnm-gtk-dev libnm-dev libnma-dev ppp-dev strongswan libdbus-glib-1-dev libsecret-1-dev libgtk-3-dev libglib2.0-dev xl2tpd checkinstall

git clone https://github.com/nm-l2tp/network-manager-l2tp.git
cd network-manager-l2tp
autoreconf -fi
intltoolize

./configure --disable-static --prefix=/usr --sysconfdir=/etc --libdir=/usr/lib/x86_64-linux-gnu --libexecdir=/usr/lib/NetworkManager --localstatedir=/var --with-pppd-plugin-dir=/usr/lib/pppd/2.4.7 
make
sudo checkinstall