Announced during GDC yesterday, the Wii U's "Nintendo Web Framework" allows developers to use HTML5, Javascript, and CSS to make apps that can run on Wii U, supporting both GamePad and Wiimote controllers. We'd guess this would be most useful for app development, given the web focus of these languages, but it would be possible to make games as well.
A more directly game-focused tech developers can use is Unity. The popular multiplatform development environment is available for Wii U as well, Nintendo revealed. Both of these technologies are advertised on Nintendo's GDC site, which also offers links to the developer questionnaire for those interested in becoming licensed Nintendo developers.