Yeah, I know you said not to mention MRTG, but it is the perfect tool for reporting things like this. As long as the device (in your case, your server) reports SNMP information, MRTG can monitor, graph, and if you get creative can also notify you of overage, etc.
You may need to setup/configure SNMP services on your box, but once it's enabled, you can easily find help on the net as far as what MIB's to configure.
The ideal situation would have you monitoring the switch port that your server is connected to. If you are colocating, the company should be able to give you port information for the switch your server is plugged into. You can use this information to configure MRTG to monitor bandwidth consumption for the entire server.
If you're still interested, let me know and I can help you out getting it configured, if that's the route you'd like to go.