Mathematics and logic
Sites about math
Logic Matters
Home of the Teach Yourself Logic guide by Peter Smith.
Jeff Miller pages
Features collections of math trivia, such as the earliest use of some terms or symbols.
Cantor's Attic
Infinities big and small. Not a lot of content, but interesting nonetheless.
Large numbers
A detailed page on the ways to build absurdly big numbers.
Joel David Hamkins
A blog about mathematics and the philosophy and the infinite.
Set Theory and Foundations of Mathematics
An online textbook on set theory, algebra, formal arithmetic, etc.
PlanetMath
A mathematics encyclopedia.
Wolfram MathWorld
Another big one.
ProofWiki
An online compendium of mathematical proofs.
Groupprops
A wiki about group theory, groups, and their properties.
Database of Ring Theory
A repository of rings, their properties, and other ring theory stuff.
On-Line Encyclopedia of Integer Sequences
Also known as OEIS, a database of integer sequences and their properties.
MacTutor History of Mathematics Archive
An encyclopedia with more than 3000 biographies of mathematicians and thousands more articles about the history of mathematics.
Metamath
A database of mathematical proofs constructed from scratch from various different systems of axioms.
Googology Wiki
A wiki dedicated to large numbers.
Tools
MathURLs
Math news aggregator.
Detexify
LaTeX handwritten symbol recognition.
MathB.in
Like pastebin except with LaTeX support.
Mathgen
Randomly generated mathematics research papers. Very funny.
The proof is trivial!
Generate trivial proofs for trivial statements.
Complex Function Plotter
Graph complex functions, including custom ones. Colorful and powerful.
Ordinal Explorer Online
Starting from some large uncountable ordinal, build a list of as many ordinals smaller and greater than it as you want.
Draw Ordinals
Visualize the ordinals up to εε0.