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:

http://www.gypthecat.com/how-to-install-mod_cloudflare-on-apache-for-ubuntu

cd /root
wget http://github.com/cloudflare/CloudFlare-Tools/raw/master/mod_cloudflare.c
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 :

http://www.turnkeylinux.org/forum/support/20110502/compiling-apache-modu...

apt-get install libtool

Hope this helps someone.