There are hidden messages in Metroid Prime 3: Corruption, from Nintendo luminaries Iwata and Miyamoto. They're not satanic or disturbing in the slightest, but still interesting:

Iwata's message:
"Hello, this is Iwata from Nintendo.
A president has to take things really seriously, even if it doesn't seem like it. It's understandable if you lose weight from stress...but I get fatter instead. Nobody thinks I'm taking it seriously."

Miyamoto's Message:
"Ah, can you hear me? This is Miyamoto.
All of you playing Metroid, can you hear me?
It's fun, isn't it?
I've also worked on many [Metroid] games, but this one is the best yet, don't you think?
Definitely play it all the way to the end.
See you!"

And who could forget the door in Assassin's Creed that looks like Optimus Prime from the Transformers?

It's a bit silly, but still cool. It's hard to know where hidden messages stop and daft Easter eggs begin... so one more demonic-sounding hidden voice before we leave you with the most juvenile hidden message we've seen. This rather sinister-sounding voice is hidden in Jam Sessions on DS. When you play a specific chord, you can hear a voice whisper something. But... as a YouTube comment pointed out, it could just be the Japanese word 'Tsugimasu' which apparently means 'next'. As in 'Let's record the next chord'. But that's no fun, is it? No, must be Satan or something.

So to the final message. There's no hidden meaning as such - it's pretty self-explanatory. Found in GTA3, you can't see it unless you fly over the stadium or use a cheat, but here's a screen for you to save you the hassle.

Yup, we felt our IQ drop a few points too.