Yale is a small (272KB) portable tool which can monitor and display details on your system's processor, hard drive and network use.
The program lives in your system tray, and a quick right-click, Show is enough to open its panel. Here you'll see your current CPU and hard drive usage, as well as your network transfer rates for all adapters, as well as a graph revealing resource use history for the past few seconds. This display also updates as you watch, of course.
Clicking on one of the network adapters reveals even more information, as the panel expands to show traffic according to its type: TCP, UDP, HTTP, FTP, UPnP, Shared Network Folders and so on. (Although, annoyingly, this won't work unless the program has administrative rights. Right-click the executable, select Properties > Compatibility and check "Run this program as an administrator" to permanently avoid this problem in Windows Vista or 7.)
Yale has a few problems. It's not too configurable, for instance: if you're not interested in CPU or disk activity monitoring, then you can't turn those graphs off. And, even more basic, if your network adapter names are lengthy then they'll be displayed over the first network transfer speed figure, making it very difficult to read.
Still, if you'd like a network monitor which breaks traffic down by protocols then it could be handy. And, of course, as it's portable there's nothing to install, no penalty for trying it out.