New 3D versions of Sonic the Hedgehog and Altered Beast have been released on eShop for 3DS. The $5.99 downloads include extra features in addition to the extra dimension, including the option for classic graphics (re: pre-flatscreen curved edges), local multiplayer for Altered Beast, and zone select for Sonic. Sega has also posted an excellent lengthy interview about these latest remakes with developer M2 on their official blog.
3D Sonic the Hedgehog and 3D Altered Beast are available for $5.99 on the Nintendo eShop.
Official Site: Blogs.Sega.com