Can anyone recommend a (free) app that I can use on my Win7 computer that will enable me to monitor the network traffic to/from all the other computers and devices (9 of them) on my LAN?

I've been experiencing really slow network downloads lately - for example, it takes many minutes to connect to Facebook, or YouTube videos constantly stop while waiting for more of the stream of video, and I'm interested in trying to see where (or if?) my LAN traffic is bottled up. Is this one of my children playing Blockland or Minecraft online? or downloading Netflix videos? or playing DS or Wii games with someone overseas... I have nominally a T1 connection. I don't care about the content so much as I'm looking to measure the volume rate of traffic and where the bottleneck(s) are. It'd be nice if there were a graphical history showing the traffic to each network node over time.