Some further information about non-Euclidean geometry:

Our handout on the parallel postulate and logic.

A few other statements equivalent to the Parallel Postulate:

Spherical geometry:
brief overview
longer treatment

Hyperbolic geometry:
introduction ("neutral geometry" means the first four postulates)
one of the most common visualizations
Escher pictures that are hyperbolic: one two
miscellaneous hyperbolic geometry links