Stop adverts on websites.

Do you hate banners, popups, popunders, and adverts on websites? You do?, good, here are some suggestions on how to avoid seeing them.

Firefox - Adblock Plus

Adblock Plus is generally the most recommended method online, so I'll go over that one first. If you are using the Firefox browser, you may have already heard of this very popular extension.

A huge advantage of Adblock Plus is the subscription feature. A precompiled list of most advertising sites is included in that list, and will automatically block them for you if you wish.
Using the subscriptions will enable you to block the ads running on nearly every site. The list can be updated automatically to include new ads that find their way online as well.
Adblock Plus can also block any annoying ad or image you find online by right-clicking it and selecting it. It's a useful tool to have even if you don't care about blocking ads.
Lastly, out of the kindness of your own heart, or because you want to test your own site's ads, there is an exception feature where ads will only show on specified urls.
One thing it also blogs is sponsored Google ads, the ones down the side and in the shaded box at the top of Google search results.

Internet Explorer 7 - IE7Pro

Don't feel left out if you're an IE user! Check out IE7Pro. This add-on contains a huge amount of features and tweaks, including a free ad blocker. You can find the usage instructions on their site and get setup in no time.

Opera - Content Blocker

Opera belongs in this article as well, as they actually have an ad blocker builtin to their broswer. The usage is as simple as right-clicking an empty spot on the page and selecting Block Content... Once you select that, a notice will come up asking you what content you wish to block. Click the ad or element you need blocked and it is done. You can read more info about it at the Opera Wiki.

Alternatives

If you would rather block the ad servers systemwide, you may want to look into blocking based on your hosts file. On Windows it is generally located in C:\Windows\system32\drivers\etc\ and uses the file called hosts. Linux and Mac users will find their hosts file in the /etc directory. A precompiled hosts list can be found here, and will work on any system.

While you're editing your hosts file, why don't you block that shock site your friend is constantly tinyurl'ing you to. Adding the line

127.0.0.1 evilsite.com

and replacing evilsite.com with the bad site you don't want to see anymore will make sure that site doesn't load for you anymore. I would suggest testing this with a more mild site, just in case your first attempt isn't that successful and you accidentally visit the bad site.