WinTools.Info NetGraph is a free and portable network traffic monitor.

NetGraph shows real-time information about the state of network traffic in a visual display format selected by the user.

NetGraph is easy to use and appears overly simple, but don't be fooled. Right click and you will find a lot of features and options. You can change the display size, theme, fonts, tray settings, and more.

Numerous tools are filed under Mode, Max Rate, Graph, Adapter, Network Usage, and more. Each of these categories has numerous tools within. You can get an idea of these in the second and third screenshots below.

NetGraph is small, unobtrusive and can be set to any size you like or in the tray. If you need to simply monitor your network activity, this might be the one you're looking for.