1. Canned meat. Extremely cheap (considering)
2. Unsolicited mail/e-mail sent out in mass quantities. Usually not even addressed by name to the person who receives it.
3. In gaming, refers to a player who throws grenades indiscriminately, many at a time. Generally an issue in Team Fortress. Although lately has been expanded to include any overused gun/combo/move/tactic/etc.
"Grilled Spam is great."
"I get over thrity spam mails a week in my hotmail account."
"Way to spam the room buddy. You throw enough nades that time?"