Stuff in Pokemon you didn't know about until recently: The thread.

I just found out like ten minutes ago, that dude ('Primo' apparently) who stands next to the PC in the Pokemon Center in Violet City (HG/SS) gives you eight new wallpapers for your PC boxes and three Pokemon eggs containing Wooper/Mareep/Slugma if you tell him secret combinations of words.

I'm no newbie to Pokemon, but I just found this out by accident while looking up where to obtain Slugma.
I've had both HG and SS since launch, and I've just found this out now. I honestly thought this guy was just another useless shit for activating Mystery Gift or obtaining new words like those people in Dewford Town in R/S/E.

I thought this was a really cool secret, it's great if you want to get some diversity in your team early on. Even though you can catch Wooper and Mareep on Route 32, a Slugma to accompany your Chikorita is kick ass.

Here's how to do it for anyone else who didn't know:

http://www.filb.de/games/tools/aikotoba?l=en