Math Resources -- GAP, mathworld.wolfram.com (formerly CRC Encyclopedia of Mathematics), WWW Interactive Mathematics Server (WIMS), Encyclopedia of Integer Sequences, Math FAQ, Euclid's Elements, Math Constants, Math Database, Inverse Calculator, Calculator's Center, Pavilion of Polyhedreality, KnotPlot, History of Mathematics, Mathematician of the day, Unsolved Mathematical Problems, Dave's Math Tables, The Integrator, Statistics, Mathematical Atlas.
WWW Interactive Mathematics Server (WIMS) at http://wims.unice.fr/ by XIAO Gang, is fantastic. There are many marvelous tools here, including a Matrix calculator (determinant, characteristic polynomials, eigenvalues), a Function calculator (provides derivatives, integrals, and graphs), Factoris (factors integers and polynomials), Contfrac (gives continued fractions), a Base converter (In ternary, Pi = 10.010211012222010211002 ... ), Ruler & Compass (allows geometric constructions), Sigma (computes finite and infinite sums), a Gallery of plots, and many math exercises.
Visual Mathematics -- Miroslav Vicher's polyforms, Andrew Clarke's Polyforms, Livio Zucca's polyforms, Fractal Extreme Gallery, Erich's Packing Center, Geometry Junkyard, Mark Thompson's Recreations, Xah Lee's Plane Curves, Michael Reid's Rectifiable Polyominoes, Walter Fendt's Java Applets, Torsten Sillke's Packings, Tom Stilson's Attractors, Graphica, Poly, Rodolfo Kurchan's Puzzle Fun, Math Magic, Crompton's Tessellations, Tesselmania, LiveGraphics3D, Symmetry and the Shape of Space, Crystals, Ishihama Yoshiaki Programs, The Knot Server .
Online Math Books -- Book Recommendations, Mathworld (formerly CRC Encyclopedia of Mathematics), Around Godel's Theorem, Games of No Chance, Numerical Recipes in C, generating functionology.
Other Sites -- Mathematica, MathSoft, Ivars Peterson's MathTrek, Golomb Rulers, Cut-the-knot, Pablo Moscato's Traveling Salesman, Kepler Conjecture, Harvey Heinz Magic Squares, 2^(2^24) + 1 is not prime, Combinatorial Game Theory, USA Mathematical Talent Search .
Math Problems -- Math
Contest (motivational questions), Hexi.