The comic book Radioactive Man vs. Muhammad Ali is a parody of the 1978 comic book Superman vs. Muhammad Ali.
The pig dressed in the suit is Spider Pig from 2007's The Simpsons Movie.
The dance Nelson practices for Krusty's show is a parody of the famous routine from Flashdance.
The 1917 Kissing Lincolns penny clearly has the motto "In God We Trust" inscribed on the top. This motto started appearing on pennies in 1909, and has appeared on all coins since 1938.