As far as I know there's one opensource client, OnvifDM.
Please pay attention that this program is not related to the ONVIF group, so you can use it as an example, but it is not the officially endoersed way to implement the protocol. The only official document is Application Programmer's Guide.
Finally, ONVIF has a developer forum where you can get lots of information. You don't need any more to be member of ONVIF to access it.