Bit of a different topic than all the other games mentioned on here, although one could say this is not really a game as it is so much more than that. But I digress.
I decided to try guitar again after 30 years, and with so much out there now such as Youtube, phone apps, etc, it is so much easier. But Rocksmith makes it even easier, simply an outstanding teaching tool.
If you are learing guitar, or even know guitar already, it is quite worth your while to check it out.Lots of neat things; Session mode is incredible.
The only downside so far is that it seems to have been discontinued by Ubisoft, but downloadable content (song packs) are available.