There are fundamentally two approaches to do what you ask.
One option is to copy all your traffic to an interface (using SPAN or RSPAN) and analyze it there using some sort of protocol analyzer like Wireshark (formerly ethereal), Sniffer, of Network Instruments Observer. These are just a a few of the available protocol analyzers. The first one is free, the other two are licensed products.
The second option is to use your existing equipment's ability to export statistical information regarding the traffic flowing through it. On Cisco platforms, this is typically done using the NetFlow feature set. The extent to which NetFlow is supported varies according to your platform. See the Netflow Service Solutions Guide (http://www.cisco.com/en/US/products/sw/netmgtsw/ps1964/products_implementation_design_guide09186a00800d6a11.html) for more information. (As implemented by other vendors the functionality is known variously as sFlow, J-Flow, etc.)
Once you have NetFlow statistics, you need a tool to analyze them. Some available tools are Solarwinds' Orion Network Performance Manager (free trial - http://www.solarwinds.com/products/orion/) and ManageEngine's NetFlow Analyzer (free trial - http://www.manageengine.com/products/netflow/download-free.html).
You can also potentially use your equipment's mini-RMON MIB capability, if your platform and images support that option. An application for analysis of the data is still required. The CiscoView Mini-RMON Manager component of CiscoWorks LMS is one such tool. It requires a full CiscoWorks implementation however as it it not licensed separately. Full RMON support is available via the NAM products, hosted in an enterprise class (e.g., 6500 series) switch.
