I have worked with Ethereal/Wireshark anytime I needed to use a protocol analyser. Pretty simple to setup, install it on the computer you are going to use. Now, if you are just looking to hear whats going on in a certain VLAN, all you have to do is plug it into a switch and configure the port for that VLAN, start up wireshark and you will see all the broadcasts going on.
If you are looking for something more indepth, you will have to do some configs on your switch. I am going to assume you have a cisco switch for the following commands.
For the computer using Wireshark
monitor session 1 destination interface g0/#
*For the number after session you can pick anything from 1-66
*Enter the correct interface on the switch your computer is plugged into.
For the Computers you are monitoring.
monitor session 1 source interface g0/#
monitor session 1 source vlan ##
* Here you have two options, you can monitor specific ports. Line 1
* You can also monitor specific VLANs. Line 2
* Make sure your session number matches up with the one you picked for your wireshark.
* You can monitor multiple Interfaces for VLANs just by adding “,” or “-” to your command line.
Hope that helps.