Is there a Skype compatible client for voice calls over WiFi? I understand that operators blocked any Skype over the data connections, but is there an alternative that works on the WiFi network?

Update: I am using a HTC/Tmobile G1 phone, in the US. However, I am not connected to any carrier, as I use it (here) as a PDA over WiFi and not as a phone.
Skype for Android has a couple versions.  Most are WiFi-only, so you would be able to use them no problem except that the G1 is not in Skype's list of supported devices, so you won't find it in the Market.  Looks like it's Verizon-exclusive.  The newest version won't run on your phone even if you grab it pseudo-legally; I'm not sure about older versions.

Skype's API agreement also prohibits other apps from making Skype calls, as far as I'm aware.  Fring used to support Skype, for example, but had to remove that feature due to threats from Skype.