I've really been meaning to check out Kansas. I like Billy Greer and his solo project with Mike Slamer, Seventh Key.
If you're a fan of Billy Greer and Seventh Key, I might also recommend Streets, Steve Walsh's 80's band with Greer and Slamer. Or for a mostly acoustic project, Native Window, which is the current Kansas lineup minus Steve Walsh, featuring Greer on lead vocals.
Kansas's most recent album,
Somewhere to Elsewhere, is great as well, and it has a song with Greer featured on lead vocals. In addition, Kansas's first 5 studio albums are must-haves, in my opinion.
Leftoverture and
Point of Know Return are the most obvious starting points.