OEIS – The On-Line Encyclopedia of Integer Sequences

Dr. Hans Walser – mathematische Homepage mit Schwerpunkt Geometrie (German)

Tilings Encyclopedia – shows a wealth of examples of non-periodic substitution tilings.

Multimagie – all kinds of magic squares (solutions, proofs, conjectures, open problems)

On the 3x + 1 problem – comprehensive page for the Collatz problem

The Number Theory Web – comprehensive page on topics in number theory

Erich’s Place – Erich Friedman’s awesome mathematical homepage

Ventrella.com – Jeffrey Ventrella’s amazing homepage showing visual math, art and more

Geometry Junkyard – David Eppstein’s huge collection of geometrically stuff

MathPages – Kevin Brown’s huge collection of mathematical themes, much better than Wikipedia

Mathematik alpha – Steffen Polster’s amazing free math software