If you are using CloudFlare's fantastic free CDN service, you might want to preserve the IP addresses of vistors to your site.
I needed this so that I could use my .htaccess allow by IP feature.
Here's the link where I found how to install it:
apt-get install apache2-prefork-dev
apxs2 -iac mod_cloudflare.c
sudo /etc/init.d/apache2 restart
This was also helpful when I ran into an error :
apt-get install libtool
Hope this helps someone.