You may or may not be aware of the Gems From the Net collection that I maintain on this site. These are a collection of hilarious articles that have been floating around the internet forever. I've recently thought of a few others which aren't articles like the others I've posted, but which are absolutely worthy of mention. I should also tell you that the majority of what follows is absurd cartoon humor, for which I have a huge soft spot.
Homestar Runner is the brainchild of the Brothers Chapp. Although it started earlier, it really took off in 2001, which is probably around the time when I really started following the site regularly. During its heyday, there were new Strong Bad Emails weekly, but recently, the site has gone roughly 4 years between updates (no, I'm not exaggerating).
Of all the jokes I cite, jokes from The Simpsons and Homestar Runner are most frequent. I've got Homestar Runner miniatures at my desk at work, and I own the site's music CD, which amongst other things has a great song called Because its Midnite by a fake hair band called Limozeen.
It's hard to say enough about how great the site is, but you should check it out for yourself. Here's the Trogdor cartoon - the one that got The Burninator started.
Before Strong Bad, there was Radiskull and Devil Doll. It was apparently an early experiment in using actual audio with Flash video, but the giant floating skull immediately captured our hearts when he threatened to kill us one by one.
Real Ultimate Power became a huge early internet meme and then vanished into forgottenness. It's so obvious today that the site is a parody. But back in the early days of the internet, it was easy to believe that a 13-year-old was serious when he announced that the purpose of the ninja was to flip out and cut peoples' heads off.
Lastly, a cartoon that makes me laugh out loud every time I see it. CRAB BATTLE! If you're familiar with the Metal Gear Solid games, you can probably relate more to the video, but I think it's hilarious any way you slice it.