Here are a few websites that I can recall from my teen years.

Newgrounds - This is where I discovered most of my favorite independent animators until they sold out on YouTube and eventually suffered burnout and left the scene.

Funnyjunk - Similar to Blacksheep and Newgrounds, with hilarious videos, games, and the lovely "Salad Fingers" shudders.

eBaum's World - Funny pictures and videos.

MINDistortion.tv - All I remember doing on this site was creating my own Pocket EMO.

Homestuck - A web comic made in MS Paint; I was obsessed with the series.

Gaia Online - When the site launched, my brother and I signed up; he deleted his account, but I still have mine and am very much still active. Customer service is pretty much nonexistent nowadays, so if you decide to delete your character, it may be near impossible at this point.

Habbo - A virtual world with avatar chat and pixel art.

OldSchool RuneScape - I still play it to this day ^_^'.

Club Penguin - Everyone played ClubPenguin. Yes, even the edgiest looking brooding goth kid who sat in the back of the classroom. Speaking of penguins "Poke the Penguin?"

Quizilla - Fanlore, fanfiction, and quizzes for fun.

FanFiction - I probably spent way too much time on this website than I'd like to admit, along with Tumblr.

SomethingAwful - Something Awful was a website with funny content, but its community was the most important part. Users created the Slenderman legend and started the Let's Play video genre.

I Can Has Cheezburger? - It's not the same; most of these websites underwent many changes and the rise of ads destroyed the fun of it all. -_-'

Hamster Dance - I was obsessed with this song. Then I found The Gummy Bear Song and Crazy Frog, and let's just say I have a pattern. But now, hearing these silly songs fills me with nostalgia and transport me back to a simpler time in my life.