transparent proxy / tool for sniffing web services traffic

Question

I'm trying to query a webservice with fairly limited documentation. I do have a working client application (no source code though), and I'd like to see what it's sending when I run it. Is there a good tool available for this? Any advice?

EDIT: the client app only runs on Windows.

---

EDIT LATER: Having had a chance to play with Wireshark (as per the accepted answer), I would really recommend anyone who is doing anything remotely network related to have Wireshark in their toolset. It's free and not that hard to get into, and (particularly for human-readable stuff like HTTP and XML) it's really interesting to get your head around exactly what is going on.

Answer 1

If you can set a proxy for the client, use Fiddler: http://www.fiddler2.com/fiddler2/

If you can't, I recommend Wireshark: http://www.wireshark.org/, it's a bit more complicated, but it can do the job

Answer 2

I'd normally use SOAPUI as a proxy between the application and the back-end service.

Answer 3

A simple to use but powerful proxy tool that specializes on Web Services is Membrane SOAP Monitor. It can also format the XML payload, you can modify and resend messages. It is perfect to analyse a undocumented SOAP interface.