Network Packets

I like to use Ethereal...and it's free.
http://www.ethereal.com/

In a switched network environment, make sure you are able to select one port for mirroring. You need it to view all packets, otherwise you'll only see the broadcast and multicast traffic, no unicast without it.
http://www.ethereal.com/faq.html#q5.1

Ethereal is free, but only offers decode and post capture statistics. I use Observer, one of the many "for charge" analyzers. The for charge products offer more statistics, and at the higher end offer problem identification through expert analysis (this is a feature that scans through captures and tries to identify problems).

Monitoring 100 systems can offer some challenges with respect to switch span ports. A span port by its nature can only send X amount of data (100 Mbs usually) and the 100 systems may generate in aggregate more data that the span port can handle. In that case the switch will drop packets.

I like to only span the port of the server in question or send all the spaned ports to a faster port than the original systems are on. For example if the original ports are 100Mbs, span them to a 1000Mbs port.