The WonderWitch was a full blown development kit with rewritable cartridge for the WonderSwan. It was produced by Qute Corporation with Bandai's approval, and it was sold via mail order only. So, it's basically the system's official flash cart. While it won't run commercial ROM dumps, there are hundreds of homebrews up on the official site, owing to the WonderWitch Grand Prix coding compos run yearly from 2001 to 2003. Many are of surprisingly high quality, and two (Judgement Silversword and Dicing Knight) were good enough to receive commercial releases on regular Swan carts. Pages for each year's winners are on the Qute site: 2001, 2002 and 2003.