Merle Haggard — "From Graceland to the Promised Land"
Rhonda Vincent — "Is the Grass any Bluer on the Other Side?" (Tribute to Bill Monroe, "Father of Bluegrass.")
Asleep at the Wheel — "Bob Wills is Still the King" (This is actually a Waylon Jennings song, but this version is done in Western swing style.)
These three stand out as being exceptional examples of this form of song, however there are two other similar songs I think are worth mentioning. One is George Jone's "Who's Gonna Fill Their Shoes?" which is a tribute to several artists. (Song starts at 1:50 if you want to skip the intro, but I like the intro.)
The other is Bob Dylan's "Song to Woody," which is a tribute song but also a Bob Dylan song.