If you've got a problem on the wire, that's above layer 2 in the epoymous OSI model, then a network sniffer may well be your tool of choice. Choosing a packet sniffer is mainly a combination of platform and preference.

For Windows, the obvious choice is Wireshark (previously Ethereal) it's both free and provides most of the functionality you're likely to need.

Microsoft provides Network Monitor (netmon) which is also a good tool, although for most tasks it doesn't substantially differentiate itself from Wireshark - you may however, prefer the user inferface. Even if you aren't interested in Microsoft's offering, the netmon blog does provide a good source of information on packet capture and analysis for the beginner.

For other platforms, we'd recommend Wireshark. But getting it installed on a Mac can prove problematic, if you don't read the documentation carefully first.

So go out there and capture some packets, ethically of course!