Science:

Optimization-online – Repository of e-prints about optimization and related topics.

COIN-OR – Initiative to spur the development of open-source software for the operations research community.

Khan Academy – Learn anything for free!

Code Academy – Start coding now!

N is a Number – A Portrait of Paul Erdős – Documentary.

Mário Sergio Cortella – Qual a postura ideal do professor? – Lecture of the professor and philosopher Mario Sergio Cortella (in portuguese).

Mathematics Genealogy Project

Misc:

Audiofanzine – Music & audio.

Deviantart – Art community.