Sega just released 3D ports of Sonic the Hedgehog and Altered Beast, two classic Sega Genesis titles, on the Nintendo 3DS eShop. Each game sells for $5.99. Space Harrier and Super Hang-On were released last week.
For more in-depth look at the making of these games, visit the SEGA blog for an ongoing series of interviews with the M2 developers at http://blogs.sega.com/category/sega-3d-classics/