There's a recent topic on Zigbee and Wifi which explains it well:
There are many other threads as well.
Wifi Channel 61 is a 5GHz WiFi channel and doesn't have anything to do with Zigbee interference on 2.4GHz.
I leave Z-wave to be answered by someone else who actually use it. 
From what I understand it is not an issue that needs to be bothered with since the frequency used is less "crowded".
