Set Theory Talks, information about set theory talks, conferences and seminars around the world. 
Shelah's archive, the list of all his publications, articles or books alike. 
Booles' Rings, an innovative website designed to encourage interaction through the web between mathematicians. 
The Mathematics Genealogy Project, an ambitious project for tracking the "genealogy" (here, the Ph. D. thesis supervisor plays the role of "father") of mathematicians. 
Logic around the World, an enormous resource database for research in Mathematical Logic. 
I am a big fan of MathOverflow, an innovative web platform for interaction between mathematicians by means of a questionandanswer format. I am actually a registered user:  
El Irracional ("The Irrational") is quite similar to MathOverflow, but in Spanish language. In it, I am an "editor" (which just means that I have some administrative privileges). 
The complete works of P. Erdös. 
The MacTutor History of Mathematics archive, containing plenty of famous mathematicians' biographies. 
My current institution: Mathematics Department, University of Michigan. 
Centre for Mathematical Sciences at UNAM, Morelia, México. 
Department of Mathematics & Statistics  My PhD is from the Department of Mathematics and Statistics at York University. 
A lot of my mathematical education happened, in many ways, at the Fields Institute in Canada. 
The Stanford Encyclopedia of Philosophy  
I really love my native language (Spanish). Here's the link to the Royal Spanish Academy of Language, and the online functionality of their Dictionary:  

Piled Higher and Deeper: PhD Comics. 
Saturday Morning Breakfast Cereal. 
Abstruse Goose 
XKCD: A webcomic of Romance, Sarcasm, Math, and Language. 