Wide-boy 64 for Gameboy Advance.

This development cartridge was used by developers to play Gameboy Advance game cartridges on a television, by using an N64 game console. Released by Intelligent Systems at a cost of $1400 USD a pop, and were only available to developers and Industry professionals. (Gaming Magazine writers/review staff)

These days you can still find them for sale on Japanese online Auctions. But will likely set you back several thousand USD Dollars. These were available for both the Gameboy color (CGB) and Gameboy Advance (AGB) cartridges.

Images Provided by @GamingLegend64 (Twitter)

This version of the AGB Wideboy64 has an added IDC socket for connecting a Gameboy handheld, to be used as a control device during gameplay.