Truly Free SSL

https://letsencrypt.org/

git clone https://github.com/letsencrypt/letsencrypt
cd letsencrypt
./letsencrypt-auto certonly --webroot -w /website/wiki.chrisjharding.com/public -d wiki.chrisjharding.com

The script will use quite a bit of temporary memory! If you get errors, try increasing ram or temporarily enabling a swap file. This script will put some files in your webroot just for validation.

Alternatively you can do it without it using your webroot (although you will have to stop your server listening on 80 and 443) The first file in the list will be sued in the directory when saved.

./letsencrypt-auto certonly --standalone -d example.com -d www.example.com

You will then need to change the nginx / apache virtualhosts to point to the certificates in /etc/letsencrypt/live/[YOUR DOMAIN]

server {
        listen 80;
        listen 443 ssl;

        ssl    on;
        ssl_certificate    /etc/letsencrypt/live/wiki.chrisjharding.com/cert.pem;
        ssl_certificate_key    /etc/letsencrypt/live/wiki.chrisjharding.com/privkey.pem;

        ... Other Stuff
}