From Sega Retro

While the concept of 16:9 anamorphic widescreen is generally considered to have become a mainstream addition to games post-Sega Dreamcast, a number of older games support this mode, despite widescreen televisions not being readily available at the time.

In most situations, widescreen support only affects the 3D portions of a game - 2D areas such as menus are usually fixed at 4:3, and so will be streched on a widescreen display.

Sega games which support 16:9 widescreen

Sega 32X

Sega Saturn

Sega Dreamcast

Note: Widescreen can be forced in more games by using Action Replay CDX cheat codes.