I believe your options in the US are limited, I know Timberline in oregon is going strong, A-basin in Colorado is also open and operational. In Canada probably Whisler is open and a few others.
Timberline is a year round 7 day a week sure thing. Right now Silver Mountain in Northern Idaho is still open and they are getting fresh pow right now. Up at Whistler/Blackcomb, there is summer riding on their glacier, but I understand it is`nt as big as Palmer at Timberline and does not run all summer.