Wii U Virtual Console right now, as part of the Trial Campaign promotion celebrating the Famicom.
Super Metroid is fourth on a list of discounts that kicked off back in February – predecessors include Kirby's Adventure, Punch-Out!!! Featuring Mr. Dream and F-Zero. Super Metroid will be $0.30 for 30 days, until Yoshi comes in and takes over on June 12.
The Virtual Console launched on Wii U late last month. The initial launch list included Balloon Fight, Donkey Kong Jr., Excitebike, Ice Climbers, Kirby's Adventure, Punch Out, F-Zero and Super Mario World. This week, Super Mario Bros. 2 will be available for $5.