http://www.betternovelproject.com/ – Examines best-sellers like The Hunger Games and the Harry Potter series to work out the common elements shared by many successful novels. I’ve also been lucky enough to have an article published on the site.
http://www.superheronation.com/ – Hundreds of free to view writing articles. A good portion of superhero-specific articles as well as heaps of general writing advice. Greatly helped increase my writing ability.
Some great pages on Super Hero Nation:
http://www.superheronation.com/2009/04/24/writing-tips-of-the-day-how-to-beat-writers-block/ – Tips to get past the dreaded writers block.
http://www.superheronation.com/2009/04/03/how-to-pace-a-scene-more-quickly/ – Excellent information on how to write action scenes in a smooth, flowing manner.
http://www.superheronation.com/2007/12/30/list-of-superpowers/ – Try to think of a superpower that isn’t covered by this. I guarantee that will be pretty difficult :). A great, comprehensive list.
http://www.springhole.net/ – Very similar to Superhero Nation. The website offers countless articles on how to improve your story, from constructing amazing fantasy worlds to getting into the minds of your characters. On top of this, the site is packed with random generators, which can be very useful when writing.
http://textadventures.co.uk/ – Hundreds of various text-adventure games, which can be played online or downloaded to play offline. My own text adventure game, Double O Zero – The Affable Agent, is available on this site.