I was exploring some of the articles on Sharpbrains.com today, and I discovered a link to a nice site on meditation.
I have been looking around for something like this, but most of the meditation sites seems to sell meditation classes, and nothing more.
How To Meditate on the other side, seems to have at least some good explanations to learn the basics of meditations. They seem to focus on Buddhist meditation, but I reckon that is as good as any… 
Here are some of the topics covered:
Breathing Meditations
Transforming Meditations
Lamrim: The Stages of the Buddhist Path
How to Meditate on Lamrim
Loving Kindness Meditation
(and more…)
