MythBusters paint the Mona Lisa with paintball guns

In-flight entertainment

If you’re sitting next to someone on a plane who irritates you, try doing this:

1. Quietly and calmly open up your laptop case.
2. Remove your laptop.
3. Start it up.
4. Make sure the fellow traveller who is annoying you can see the screen.
5. Close your eyes and tilt your head up to the sky and move your lips like you are praying.
6. Then click here.

Source

Watchmen parody

Nostalgia Critic: Captain Planet

Source: That Guy With The Glasses

Warhammer 40k

And all of it's tropes... (which are a lot)
If you don't know 40k or tropes, this is a good a place as any to start.

One of them gives a good description of the universe and the associated tropes:
Trope Overdosed (We reiterate: 40k contains examples of damn near everything, most of it trying to kill you.)

Wikihistory

This is what will happen if we invent time travel...