Sega’s released Sonic & Knuckles on RealOne arcade. You can download an hour free trial of it via this page, which also shows the system requirements you’ll need to run the game. If you don’t have RealOne arcade, it gets downloaded onto your hard drive. You can buy the game for $14.99, which gives you unlimited access. According to Sega.com you can get the lock on extras for Sonic 2 and 3, but this may be only by the paid version, or not actually a feature of the RealOne version of the game.
Sonic 1, 2, 3 and Dr. Robotnik’s Mean Bean Machine are already available on RealOne arcade, and Sonic Spinball will apparantly be released later this month.
Sega have also released some Game Gear games for the Tungsten T palmtop, including the 8 bit versions of Sonic 1 and Dr. Robotniks Mean Bean Machine. This page has all the details.
Source: Sega.com