Re: Obvious references to other songs
The lyrically, most obvious are, as mentioned, Sweet Home Alabama that responds to Neil Young's "Southern Man."
And the 80's new wave act Peter Schilling's song "Major Tom" was a continuation of David Bowie's "Space Oddity".
Dream Theater cross-reference their own songs all the time, and often slip in little riffs or lines that play tribute to other artists songs.
Rush have been known a few times to have a song on one album that continues a theme of a song from a prior album. Such as their "trilogy of fear" and the instrumental song "Where's My Thing" has a similar title to an instrumental a few albums later "Leave that Thing Alone".
The Offsprings "Get a Job" is from the Beatles' song "Ob-La-Di, Ob-La-Da".
Led Zepplin's Boogie with Stu is from Richie Vallens "Oh, my head".
The one I've never understood why there isn't song writing credit given, is the bridge on Bryan Adam's "Run to You" is the same riff as Journey's "Send her my Love."