Other
Succession to the Presidency a Chronology
This site is put together by a medical doctor with an interest in the line of succession to the presidency. Although not a constitutional expert, the author has documented source material and has read extensively in the field.
Annenberg Foundation
Annenberg Classroom: Twenty Fifth Amendment
From the Annenberg Classroom Consititution Guide, provides an overview of the text and meaning of the 25th Amendment.
Annenberg Foundation
Annenberg Classroom: Presidential Succession
This website contains an interactive timeline about the history of Presidential succession.
Siteseen
Siteseen: Government and Constitution: 25th Amendment
Short, simple summary with the full text of the 25th amendment detailing changing what happens if a President or Vice President dies, resigns, or is unable to do the job.