When I was browsing on commandlinefu.com, I saw this entry Block the 6700 worst spamhosts: (URL edited for plain text file)
wget -q -O - http://someonewhocares.org/hosts/hosts | grep ^127 >> /etc/hosts

As of writing (2012-03-24T08:01:18Z), the list, made by Dan Pollock, has grown to 9,502 domains. That is insane! See how many spam websites we have, although not all are spams, some of the entries are legitimate advertising distributors.

To be honest, I was really tempted to use it, but the huge amount of entries did hold me back completely.

If you want to try it, I can propose you a short script as system cron task. I didn't test and I am writing in on the fly, so use it as your own risk:
cd /etc
# just in case, you haven't saved current hosts as hosts.local
[[ ! -f hosts.local ]] && exit 1
if [[ "$(curl http://someonewhocares.org/hosts/hosts -z hosts.hosts -o hosts.hosts -s -L -w %{http_code})" == "200" ]]; then
  cat hosts.local hosts.hosts > hosts
fi
You will need to run as root first
cp /etc/hosts{,.local}
The script will concatenate your current hosts and the one downloaded from the website. Set up a daily cron task for it, it will only download the file when the files get updated, the method is described as in this blog post.

Be sure to read the comments on the website, which also provides some different modifications and even a RSS feed for notification.