- Abstract Algebra Online
Contains many of the definitions and theorems from the area of mathematics generally called abstract algebra. Intended for undergraduate students taking an abstract algebra class at the junior/senior level, as well as for students taking their first graduate algebra course.
- IDA: Algebra Interactive
An HTML-based book by Arjeh M. Cohen, Hans Cuypers and Hans Sterk. It focuses on an algorithmic approach to algebraic structures. Many of these tools and examples use the computer algebra package GAP. Site has samples and errata.
- Home Page of J. S. Milne
Includes preprints and course notes on Group Theory, Fields and Galois Theory, Algebraic Geometry, Algebraic Number Theory,Modular Functions and Modular Forms, Elliptic Curves, Abelian Varieties, Etale Cohomology, and Class Field Theory.