SHow INTerface will give you traffic stats, including errors and broadcast frames/packets.
Some/most/all (recent) Cisco equipment (varies with OS version) may also include a useful http server process that can display traffic stats per interface.
PIX PDM can show you interface utilization stats.
SDM (~800, 1800, 2800 ...routers) will give you per-port stats & utilization graphs.
MRTG (free application / SourceForge) can monitor the interfaces via the MIB and generate dynamic web pages with utilization/stats.
FWIW
Scott