If you like browsing the web for cool stuff, this thing is for you.
StumbleUpon is an add-on that works with Firefox and (just recently) Safari. It allows you to find webpages that might interest you based on what you like. People who use StumbleUpon can also submit sites they find so that they are shared with other users.
I've played around with lots of add-ons. Most of them are cool for a bit but are then soon forgotten and left alone. StumbleUpon is the one add-on I actually like to use.
To get started, download StumbleUpon:
Safari Version: http://www.apple.com/downloads/macosx/internet_utilities/stumbi.html (Shareware)
Firefox Version: https://addons.mozilla.org/en-US/firefox/addon/138 (Freeware)
Once you have the plugin you need to register an account with http://www.stumbleupon.com/. On the site you are asked to enter all your interests. You can also edit your profile along with other things.
Once your accounts the way you like it, login into it using the toolbar (Tools > Sign In). Now your all set.
Now whenever you want to find a new cool site click "Stumble" and you'll get sent to something related to your interests. Also, if you are ever browsing and find something cool, click "I Like". If it has never been found before you get to submit it to StumbleUpon.
Enjoy, it's a fun way to kill time!