For a subset of Cisco's switches, the free product Cisco Network Assistant tool can help. Also, some switches have a built-in Web UI that allows one to see near real time performance (utilization) on a given port.
To monitor a large set of switches' traffic, have a look at Cacti, an open source tool.
Remember though - cost is not just cost of acquisition. Often getting something "free" costs you more in operational costs than the "expensive" (to buy) product.
