Theoretical

Church-Turing Thesis
Wikipedia article.
http://en.wikipedia.org/wiki/Church-Turing_thesis

Computability Logic
Main web source on the subject: overview, downloadable articles, links.
http://www.cis.upenn.edu/~giorgi/cl.html

Computability Theory
Wikipedia article.
http://en.wikipedia.org/wiki/Computability_theory

CSCI 0510 Models of Computation
Undergraduate computer science course on the foundations of computing at the Brown University, Department of Computer Science.
http://www.cs.brown.edu/courses/csci0510/

Interactive Computation
Wikipedia article.
http://en.wikipedia.org/wiki/Interactive_computation

Introduction to Formal Models in Computer Science
Lecture notes from the University of Washington covering basic topics such as computation models and theoretical concepts such as nondeterminism.
http://www.cs.washington.edu/education/courses/322/06sp/

Lecture Notes on Theory of Computation
Introduction to basic topics including models of computation, finite automata, context free grammars and complexity theory.
http://www.jn.inf.ethz.ch/education/index.html

Rusty Spigot Computer Science Notes
Notes on a range of computer science subjects, from the Cambridge undergraduate course.
http://www.rustyspigot.com/Computer_Science/

Theoretical Computer Science Search Engine
A Google Custom Search Engine that searches pages relevant to Theoretical Computer Science, such as books, lecture notes, courses, and papers.
http://searchtcs.googlepages.com/

Theory of Computation
Lecture notes for an undergraduate class taught by Leonid Levin at UC Berkeley.
http://www.cs.bu.edu/fac/lnd/toc/