Flash games are fun. Some are addicting (albeit brain rotting). For instance, snake. Who hasn't enjoyed a good game of snake? For those of you unfamiliar with snake, it's basically a game where you start out as a dot, and you have to eat food (more dots). The more you eat, the longer you get. If you crash into yourself, or the walls, you die. There are a lot of versions floating around the internet right now.
Another one of my favorites is Jumper 3 (it's not about suicide, despite the name). Basically, you jump over this circle from plank to plank. The more you avoid jumping into the circle, the more points you get. If you go through the dot, you lose ten points. It's better than it sounds.
Also, there's Robot Unicorn Attack! It was created by the folks at Adult Swim. This is the cycle of addiction for this game: 1. You're wondering what drugs the creators were on. 2. Approximately two minutes after 1, you start to realize just how much fun it is. 3. Five minutes after one, you start singing along to the song.
The point of the game is that you're steering a, well, Robot Unicorn (you have to cap the first letters of robot and unicorn. It is a must) through a magical land. Z is to jump, X is for rainbow attack (to explode stars!) If you fall off a floating rock, or run into a star, you die in a fiery explosion. There's dolphins, a stupid pop song, rainbows, rainbow-fairies, and explosions. It's almost too much fun.
Now, for the grand finale, the most time-sucking way to procrastinate online... Tv Tropes. It's a wiki that has a whole bunch of "tropes" that show up in fiction. Tropes are, in essence, elements that appear in fiction works. You know the bad guy everyone loves because they don't know he's bad? Bam! That link is the trope. You know that one guy that has no emotions? Bam! Have you ever seen a hero do something so ridiculously stupid you want to smack them? Bam! How 'bout if you've seen a movie, or read a book, and want to see what tropes are in it? Just go to the top right of the page and type it in, and click on the main article. If you have a question about a work, feel free to add it to the work's Headscratcher's page. Never mind that it will suck up all your time like a black hole, it's just so much fun to read.
So if you have too much time on your hands, just google the games mentioned above. If you have a day to spare, click on any of those links above.