Cacti is a good open source solution. You will need some Linux experience to get up and running.
PRTG is a nice solution, can't be the price. You can use SNMP to monitor interfaces and it now also includes Netflow. Probably not the most robust reporting, but a good starting point for the price.
SolarWinds is another huge player. Great features and reporting. Only downside is you will pay a higher cost. IMO, the benefits justify the cost, but every scenario is different.
ManageEngine has some nice products, but I have always found their interface and navigation not as intuitive as other solutions on the market. Either way the product does what is advertised.