Special Interest Group on Algorithms and Computation Theory, the ACM special interest group for Theoretical Computer Science. Site has membership information, meetings, reports and a newsletter for members.
An initiative of attendees of the 1997 Dagstuhl seminar, these pages provide research papers, a bulletin board, and links to researchers and other resources in the field. The focus is on average case and probabilistic analysis.
A dictionary of algorithms, algorithmic techniques, data structures, and archetypical problems, with related definitions. Many entries have links to implementations, tutorials, and bibliographical references.