well let's see. you've got Myspace and Tagworld for all the teenie boppers and early 20 year olds. well, pretty much everyone is on myspace. it's pretty cool, and I'm sure you have a myspace. searching for anyone you've ever known is quite fun.
www.ebay.com is a fun website for selling, trading and buying things. you can find ANYTHING on here, you could even buy a fish skeleton or a piece of toast (as well as a car or kitchen cookware set) on ebay. all you have to do is find it. I think i'll sell some things on e-bay this summer.yay for making money!
Google is the BEST search engine website, you can search for anything and find a valid answer to any question you have, can research lots of info, essentially you can find anything you want on google.
Neopets.com is a fun virtual pets site, and you would be surprised how many ADULTS are on neopets. my little sister loves neopets and all of her friends find it wonderful. (she's 12.) Neopets has a lot of fun games to play and some other cool stuff about it to, so if you're bored and have a few hours to kill, www.neopets.com is a GREAT place to waste time hehe.
coolbroadbands (you can search it on google!) is a site where you can find all sorts of broadband videos to watch, pretty much on any subject you look for. along those lines, there's another site called youtube.com that has videos of all kinds (music videos, comedy/humorous videos, etc.) that are submitted by the viewers of the site. Anyone can submit videos, and you can see what the people of america are up to. don't be surprised if you find something stupid to watch.
then you have your typical news websites that update information and world happenings, like Yahoo! and MSN.com. if you like keeping up-to-date on current news stories, check out msn.com and read some articles.
If you have a mac or just want to check out a pretty neat website, go to apple.com and they have lots of stuff to read and watch. they have quicktime movie trailers of almost every movie coming out in the near future, so if you want to see what's playing WAY ahead of time, check that out.
teagames.com has some cool online games to play too.
some of the most popular things being searched for on the web these days are the NFL draft, Cinco de Mayo, illegal immigration practices, David Beckham, Ana Nicole Smith, NASCAR, and the Darfur region of Sudan. go to google and search for any of these topics, and you'll probably come up with a preeeeettttty popular website :)
needless to say, porn is always popular, so you have a huge number of porn website on the net. great huh.
good luck, I bet this was for a homework assignment, huh hahah. if not, have fun with it anyway!