Solution Methods

Method by Lars Petrus
An efficient method based on intuition - incremental block building.
Gilles and his cubes
An interesting efficient method - the mix of Lars Petrus's and corners first.
Method by J. Fridrich
The most famous method nowadays directly from the source. F2L, OLL, PLL terms refer to this method.

Important and/or Interesting

Speedcubing.com
The home page for all speed-cubists.
Jaap's Puzzle Page
Most comprehensive site with solutions to many puzzles.
MagneticCube.com
Magnetic dice cubes work like a Rubik's cube, but are made entirely of dice and magnets.
Cube Stuff at Wunderland
Kristin and the Cube, Rubik's Cube Art, Custom Cube Technology, etc.

Cube Art

Cube Art at Wunderland
Mostly 2D pictures made from Rubik's cubes
Fred Holly's Rubik Cube Design
Very nice symmetric 2D patterns from Rubik's cubes

Cube Tools

Interactive applet
Allows to convert move sequences to an animation (using AnimCube applet).

For more links go to Speedcubing.com links section