What is your favorite language resource on the web?
Where do you go to find correct usage? Spelling? Inspiration?
What tools should people be aware of? References? Translators?
What other interesting language resources are out there?
Guidelines:
- One resource per entry (but add all the good ones separately!)
- A quick description of the resource would be helpful
- Add an example or two if appropriate
- Please indicate when resources are non-free
- Check for your entry first, and up-vote it if it exists

This is intended to be a community moderated list of resources. I intend to choose a ‘correct’ answer from the highly rated results (that are not my own) when it has had time to grow.
iRosetta.com for the win :D