I use Firefox and all links here are for the firefox browser. Chrome users can search for these same addons in Chromeland.

Abine (Blur)     https://addons.mozilla.org/en-US/firefox/addon/donottrackplus/

Addon Blur has become particularly irritating since being purchased from abine but it still works. You need to create an account for some reason now (I use throwaway email like mailinator) but you also need to modify it to prevent the constant reminders about how good a job its doing for you. To do this, simply install blur, register it with a junk email account, then click on settings and uncheck everything except "show medals" (to see how many sites and times you've actually blocked tracking). Blur does a fine enough job and runs intrusion detection just fine without you seeing all the popups blur itself creates.

Adblock Plus   https://adblockplus.org/en/

A must for any Firefox or Chrome user. If you're using Internet Explorer for any other reason than an older webpage designed for it only like many state or federal government sites, shame on you!

Flashblock       http://flashblock.mozdev.org/

Amazing! Works! Will stop all animated adds cold! You'll notice a strange symbol of a circle with a lowercase " f " where the flash ads used to be. If you are dying to see the ads, click on the "f" and they'll run for you.  Youtube is affected by this but in a good way. When you click on a youtube link, it won't start playing immediately, if now waits until you click on the symbol. For me, this is beneficial.

Behind the Overlay  https://github.com/NicolaeNMV/BehindTheOverlay

A newcomer that works fine for these "new" type and most irritating popups generated by the website itself. The ones that cover 75% to 95% percent of the screen when you open a webpage. The type that cajole you to sign-up for a subscription to the site itself or want you take a some sort of survey or sell you the sites latest features.  With this addon, they are GONE!