Does anyone know of a simple way of showing which of my machines on my home network is hogging all my data?

I recently discovered that something on my network is using ~30M per hour all day and night and I'd like to find out what it is. I guess I could go around and turn each thing off and check my ISP's usage meter over a few hours, but there must be a better way of doing it.

I have a WRT54GL running:

Firmware              OpenWrt Kamikaze - With X-Wrt Extensions 8.09
Kernel          Linux 2.4.35.4 #15 Fri Jan 22 11:36:55 CST 2010
MAC          00:18:39:ED:A2:73
Device         Linksys WRT54G/GS/GL
Board         Broadcom BCM5352 chip rev 0
Username          root

Web mgt. console          Webif²
Version        r4838

I'm very lazy, so an installable package that can do this over a web interface would be ideal, but not necessary.

anyone?