Who's On My Wifi can detect any device that communicates over a TCP/IP network using an Ethernet or Wireless connection.
The following is a list of devices that can be detected, but is not limited to:
Desktop Computers - Windows, Mac, Linux machines will all be detected
Laptops - Windows, Mac, and Linux machines will all be detected
Servers - Windows, Mac, and Linux machines will all be detected
Game Consoles: Wii, Xbox, PS3, if they are connected to the network
Home Electronics: HDTVs, BluRay players, if they connect to the network
Smart Phones: Iphone, Android, Blackberry, etc. if they connect over wifi instead of connecting through the cell phone companies data connection
Reading Devices: Kindle, Nook
Network Devices: Firewalls, Managed Switches, Managed Routers, etc.
Specialty Hardware Devices: Video Recorders, Clocks, SAN disks, etc.
Basically, if it has an IP address, and it's on YOUR network, it can be detected.