The SEGA AGES Online titles include games from the Master System, Genesis, and the SEGA Arcades. Titles such as Alex Kidd, Streets of Rage, Super Hang-ON (with optional 3DTV support on Xbox 360), and Monster World IV (in English). Check out the full release list: here.
I only toyed around with it a little bit today but I was very impressed with what I saw on the Xbox 360. The pricing on these is a bit scattered. On PSN each title is available individually for $4.99 while Xbox has packaged groups of titles for 800 pts.
I highly recommend the Monster World collection.