RSS reader with adblock
I've added this rule to the end of /Applications/Reeder.app/Contents/Resources/article.css:

iframe[src*="feedads"],
iframe[src*="doubleclick"],
iframe[src*="plusone.google"],
a[href*=".ads."],
a[href*="feedads"],
a[href*="feedburner"],
a[href*="doubleclick"],
a[href*="//ads."],
a[href*="api.tweetmeme"],
a[href*="delicious.com/post?"],
a[href*="digg.com/submit?"],
a[href*="google.com/bookmarks/mark?"],
a[href*="posterous.com/share?"],
a[href*="tumblr.com/share?"],
a[href*="linkedin.com/shareArticle?"],
a[href*="facebook.com/share.php?"],
a[href*="http://twitter.com/home?"],
a[href*="addtoany.com/share_save"],
a[href*="twitter.com/share?"],
a[href*="pheedcontent.com/"],
a[href*="feeds.wordpress.com/"],
img[src*=".ads."],
img[src*="//ads."],
img[src*="doubleclick"],
img[src*="feedads"],
img[src*="feedburner"],
img[src*="share-buttons"],
img[src*="pheedo.com/"] {
    display: none !important;
}


Most of the selectors were taken from a CSS file posted by the original developer of NetNewsWire.

You could also block domains that serve ads in /etc/hosts.