Meet the GIMP! (http://meetthegimp.org/) A videopodcast about the Gimp, great place to learn it. A must for all that claim Photo$hop is the only one pixel manipulation program.
lensfun library (http://lensfun.berlios.de/) Project dedicated to lens calibration.
Tethered shooting on linux (http://photodoto.com/tethered-shooting-with-linux/) Article explains how to shoot tethered using gPhoto2 (does not work with all cameras)
open printing (http://www.linux-foundation.org/en/OpenPrinting) Check it out if you are interested in printer support or other printing connected subjects.
open source photography wiki (http://wiki.osphoto.org/index.php/OSP_Wiki:Community_portal) Wiki dedicated to open source projects.
Photography by Open Source (http://opensourcephotography.org/) Blog about photography and open source software.
Linux photography blog (http://jcornuz.wordpress.com/) Self explaining.
Linux and RAW Digital Photography (http://www.galacticslacker.com/Linux_RAW.shtml) Wiki dedicated to open source photo applications.
CHDK wiki (http://chdk.wikia.com/wiki/CHDK) Alternative, unofficial CHDK firmware for Canon's Digic II and Digic III cameras.
Remote control driver for Nikon cameras (http://www.math.ualberta.ca/imaging/) Alternative, unofficial libraries to remotely control some of the Nikon cameras (some other vendors are reported to be compatible), linux version available.
Mounting digital cameras under linux (http://users.bigpond.net.au/blmiller/linux/c5050.html) Digital cameras (as well as card readers) are automatically detected and mounted by all moder linux distributions. For those who encounter problems or want to digg into details - this page explains everything.
http://beam.at/aenima Photography related shell scripts.
easyRGB (http://www.easyrgb.com) Great place to learn about the colors and find mathematical expressions for converting colors.
Bruce Lindbloom (http://brucelindbloom.com/) Another website with colorspaces descriptions.
blend modes (http://www.pegtop.net/delphi/articles/blendmodes/intro.htm) Great comparison of blend modes (formulas included).
Poynton's Gamma and Colour FAQs (http://www.poynton.com/notes/colour_and_gamma/) Technical and very informative definitions.
Cardboard lens hoods (http://www.lenshoods.co.uk/) Did you ever lost your lens hood? Maybe you are afraid that next experiment/trip will destroy it? Visit this website, download the right pdf, print it and do your own lens hood. Spare money and have a great fun!
Do it yourself (http://www.diyphotography.net/) Interesting tips for all that do not want to spend their money on expensive equipment and are open for new ideas.
Photozone (http://www.photozone.de) One of the best websites with lens tests as well as other intersting information.
dPreview (http://www.dpreview.com/) Forum and set of extensive camera tests. Latest news from digital world.
Norman Koren photography (http://www.normankoren.com/) A lot of usefull information about technical aspects of digital photography.
Cambridge in colour (http://www.cambridgeincolour.com/tutorials.htm) No need to buy expensive books or browse forums archives. This website provide all the answeres, from bit depth to panoramas, in short and informative articles. Great place to learn for all digital photographers.
shortcourses.com (http://www.shortcourses.com/) Short and informative photography lessons.
lighting essentials (http://www.lighting-essentials.com/) Wonderfull website! All photographers that try to use flash should watch the tutorials and read the whole site. Not for beginners (still I suggest to take a look).
Planet Neil (http://planetneil.com/) A real compendium of flash techniques, photoblog, great wedding photography.
Flash photography techniques (http://www.strobist.blogspot.com/) Blog dedicated to flash photography, tutorials, ideas, articles.
ArtistX (http://www.artistx.org/site2/) A debian based live distribution shaped for artists.