Best for the two-minute laugh.
For the meeting gap, the elevator ride, the moment you need to break a streak of bad news with something stupid. These deliver in under two minutes and let you close the tab with no commitment.
Eelslap — drag your mouse to make a fish slap a man's face. The genre's purest one-second joke. Loops forever. Has been online and unchanged since 2008.
Endless Horse — a horse with infinitely long legs. Scroll. Keep scrolling. The joke gets funnier the longer you commit. By the eighth scroll you'll be laughing at yourself.
The Onion homepage — read the top three headlines, close the tab, you're done. The Onion's headline-only format is the genre's masterclass; Onion writers spend more time on the headline than the article and it shows.
FML — read four short user-submitted misfortunes from the daily list. Two minutes, four laughs, back to work.
xkcd — one comic. Hover for the alt-text bonus. Ninety seconds end-to-end.
For more in this register, the Parodies & Gags and Useless Websites categories deliver reliably.