xb90 from Posh Tips has written a gorgeous masterpiece; BgPing.

Description from Posh Tips:
“BgPing.ps1 is a Background Ping Script capable of pinging enormous hosts lists in record time. You could call it a Bulk Ping Script, a Mass Ping Script, a Batch Ping Script or even a Mega Ping Script but the bottom line is: this baby will crank through a lot of IP addresses (or hostnames) very quickly.”