After getting fed-up of not being able to read sites in Reeder on my Mac without getting bombarded with ads I looked around for a good hosts file I could use to block the buggers at the network level, rather than rely on plugins such as AdBlock at the application level.
I tried a couple, but the file available at Someone Who Cares was the most comprehensive and most up to date I could find.
So what does it do? Basically it takes a (huge) list of known ad/spy/malware domain names and tells your computer their IP address is 127.0.0.1, i.e. your own computer, rather than their actual IP addresses, so their scripts/ads/viruses/trackers/etc won’t ever load. As a side effect, ad-enabled websites should load faster.