Northern Arizona University
Northern Arizona University: Sojurn: Anatomy of a Fugue
A wonderful site with detailed descriptions of most of the aspects of a fugue, including some examples. The article gets very technical so a background in music theory would be helpful before exploring this site.
Northern Arizona University
Northern Arizona University: Sojurn: How to Analyze a Fugue
This page is very helpful in analyzing and understanding fugues. Great site for music theory students and teachers.
Other
Analysis of Bach's G Minor Fugue (Bwv 861)
This site is an analysis of Bach's G minor fugue. Also available in Spanish.