Two Free Browser plugins everyone should be using.
Both are open source, multi browser and multi-platform and best of all Free.
These will not block all ads, only the ads that try to track you. If there is an advertisement that does not try to track your information both packages will let the ad be displayed.
Blocks Ads, Scripts, iFrames, and elements.
Blocks spying attempts from websites you are browsing.
It does not block all ads, only the ads that track you. If an ad does not use tracking it allows the ad.