What does the little in-game icon mean next to your summoner name?

It's a bounty indicator

That means that player has a bounty (shutdown when killed).

you receive it for being on a kill-streak, just shows the enemy time how much gold they will receive by killing you, just a Bounty Gauge.

means killing them grants up too 450 g split between your team? i think.

killing spree. you are worth more then 300 gold and give gold to the entire enemy team when you die.