Electrum (from version 2.7.0) features support for the Ledger Nano S (Linux, Windows, OS X).
Download and install Electrum from the official site.
Select "Standard wallet"
Connect your Nano S to your computer, enter the PIN and select the Bitcoin app.
Then select "Use a hardware device"
Once your Ledger Nano S is detected:
Press "Next", and select your "Account Number". If you are not sure, leave the field with "0", otherwise enter the index of the account you want to import.
Click on "Next", and Electrum will synchronize your account. Depending on your connection and the server, it may take from a few minutes to half an hour.
Once the synchronization process is finished, you can use Electrum normally.
Sending a transaction
When you want to sign a transaction, you must validate it on your Ledger Nano S.
Important archlinux user note
Installation of python2-btchip is required to detect the Ledger Nano S:
pip3 install btchip-python
Important Ubuntu user note
These additional steps are required on Ubuntu 16.04:
apt-get install libusb-1.0-0-dev
apt-get install libudev-dev
ln -s /lib/x86_64-linux-gnu/libudev.so.1 /lib/x86_64-linux-gnu/libudev.so
pip install btchip-python