My favorite pages:
Computability theory
Computational complexity theory
David Hilbert
List of algorithms
Paradox
Quantum computer

Pages to which I've contributed significantly:
Ansible
Binary tree
Blum Blum Shub
Bohm interpretation
Boolean satisfiability problem
Burrows-Wheeler transform
Byte
Chomsky hierarchy
Completeness
Complexity classes P and NP
Computation
Context-sensitive language
Conway's Game of Life
David Hilbert
Decision problem
Discrete math
Distance
Formal grammar
Formal language
Gödel's incompleteness theorem
Grammar
Great circle
Grovers algorithm
Halting problem
Horner scheme
Infinity
Interpretation of quantum mechanics
Klein bottle
Kruskal's algorithm
Knapsack problem

Law of non-contradiction
Law of the excluded middle
Language
List of algorithms
Minesweeper
Minimum spanning tree
Mnemonic major system
Mutual recursion
Noble gas
NP-Hard
O
One-time pad
P
Pangram
Peano axioms
Penrose tiling
Prime number
Principle of bivalence
Prisoners dilemma
Pseudorandom number generator
Quantum leap
Quasicrystal
RC4
Roger Penrose
RSA
Surreal number
Tiling
Time travel
Turing machine
Turing test