Mindfulness Exercises is one of the best online sites for mindfulness-related resources. Browse the site to find free worksheets, meditations, talks, ebooks, nature sounds, research, news, image and quotes.
Greater Good: The Science of a Meaningful Life is the site of the Greater Good Science Center at the University of Berkeley. The site includes both articles and free resources on mindfulness, as well as other topics including happiness, gratitude, empathy and altruism.
Self-Compassion is the site of Kristin Neff, a compassion researcher. Her site includes some wonderful articles on self-compassion, free guided self-compassion meditations, and other practical exercises for developing greater self-compassion.