Mark Denney - many excellent videos and a very good newsletter - The Morning Blaze

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Mads Peter Iversen - very insightful blog and YouTube videos

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Photoshop - Colin Smith

Lightroom - Julianne Kost 

Both the above have free tips and tricks to help improve your post-processing skills and add to your knowledge.

 Click on this link for a table compiled by Doug which highlights the pros and cons of each.

Matt Kloskowski - an excellent video presentation for B&H video

B&H Event Space - free videos from world-class experts in all genres

Adorama - free learning resources from beginner to expert on most aspects of photography.

John Caponigro - a philosophical approach to photography and life. Many practical tips and techniques.