Here are a few of the codes I have got. They were originally created for Platinum, but I changed them to work with SoulSilver (and I shall assume, HeartGold as well). Credit goes to the original person, who I can't remember.
For all the following: Press L+R outside any Pokemart. Talk to the Green Hat Delivery Man (make sure you have room in your party)
Goon Scizor:
Crown Raikou:
Crown Entei:
Crown Suicuine: