Principia Mathematica

Mark Chu-Carroll points to the Principia Mathematica online. Chu-Carroll's summary of it in the comments is very fitting:

It is truly both magnificent and hideous: magnificent in that it's one of the most amazing applications of human intelligence, reasoning, and abstraction ever written; hideous in that it spends hundreds of pages if an ultimately futile effort to try to get to the point where the simplest statements of arithmetic become meaningful without allowing contradictions.