Firefox: ShareMeNot is a simple Firefox add-in that effectively blocks social network buttons–such as Facebook’s Like button–from tracking your activity as you browse the web.
Install ShareMeNot, restart Firefox, and from then on every web site you visit that has social network sharing buttons (such as the sharing buttons for Facebook, StumbleUpon, Twitter, Google, Digg, etc.) will no longer be able to track your activity on that page. Buttons will become active only if you click on them (or if you right click on the ShareMeNot icon and grant explicit permission to that particular page).
ShareMeNot is a free add-on and works on Firefox 3.6 and above.