I love the title
Anyway, one day when I was looking through my collection of gamecube and wii games I suddenly realied there were little triangles on the case:

Notice the green triangle at the botton of the cover? What are they?
I've been asking many people about this. Someone in a video game store told me they were for the japanese market, and that Nintendo used them to separate genres. But that can't be right. Since when is Sonic (above) the same genre as Medal of Honor... and Mario Kart (GC)?
Worse yet, here's another Sonic case, this one from the Netherlands. It's... different from the UK one (the one I first posted).
Here's a list of some games I have (this for gamecube), all labeled green:
Mario Kart
Melee
Soul Calibur 2
Metroid Prime 1-2
Star Wars: Rouge Squadron: Rebel Strike
Mario Power Tennis
Red Faction
Zelda: Wind Waker
Sonic Mega Collection.
I find it quite disturbing that all those games are from different genres, have different age recommendations (Mario tennis (3+), Soul Calibur and Red Faction (16+)). Other titles such as Prince of Persia (16+) is labeled Lime, and Fifa 02 (age reccomendation unknown since they put a fifa label on top of it, and I can't be bothered with peeling it off) is labeled Red. Also, it shall be noted that all of these games aren't 1st or 2nd party titles, right? Exclusives? No, I don't think so...
Another thing that bugs me, is that the wii seems to follow a different labeling of triangles. Most titles are -I believe- labeled purple, including Twilight Princess, Metroid Prime 3 and Super Mario Galaxy. Should be noted that Wii Sports is also labeled purple. And here's another interesting label: Purple for Wii Point Card, which isn't even a game!
Should no one here know of this, I shall take the matter further and ask Nintendo themselves. I still wanted to share it with you people first, as I assume many are unaware of this mysterious activity that is the Nintendo Triangles.