I've downloaded Adobe Lightroom in order to learn RAW image processing. I want to practise on some photos that covers basic subjects (Histograms, HDR etc).
Is there a good, free source of RAW images online to practise on?
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Check out Fro Knows Photo.
There is a weekly RAW file you can edit and you can post your result on the forums (they are at edit 81 at the time of writing). Jared (the guy behind the site) then selects a handfull RAW edits from the forum and comments on them in a youtube video. As a plus, Jared and/or Adam will give a full tutorial (again youtube video) on how they edited the RAW file themselves in Lightroom
Definitely good learning material on that site (although not always that well structured).
What kind of raw files are you looking for?
Basic shooting mistakes you can fix? - you can easily make those yourself, you said in a comment you have a 550D, so just shoot some random object with different exposure compensation values (like -3 to +3 in one stop increments), shoot a lot of sets of different subjects to get a feel of what works for what subject/situation.
Simple basic corrections? - set your camera to P mode, enable auto ISO, auto white balance and auto everything (but not the green auto mode because that will not produce raw files) and snap some pictures (preferably of people) this should give you plenty of pictures that are very close to technically correct but need some adjustment (and you don't care if they are badly composed).
Real world missed shots you can fix and good shoots you can turn into great pictures? - those are more difficult to shoot yourself, try to find a local event photographer that is willing to give you raw file to practice on (you probably need to become friends with the photographer and agree to any terms he/her has before you will get the files).
HDR, special effects, etc. - either find a local photographer who shoots the style you want to retouch or shoot them yourself
Alternatively, you can try to find a good lightroom course/workshop that provides source material to students
There are a small number (11) of freely licensed raw files available at Commons Archive, a site that I started - it hosts raw files of photos found on Wikimedia Commons, which are often used on Wikipedia. Currently there are only a few users, but I'm going to upload more there in the future and hoping to get more contributions from others.
http://rawsamples.ch has 230 files from various camera manufactures. Please add to the collection if your camera isn't represented.
RETOUCHBOX offers 9 free RAW files for you to download and use for education usage (amongst other things), as well as having a huge range of premium raw files.
You can find a lot of potrtait/model RAWs at this ModelMayhem Forum section Challenges, Contests, and Samples.
Reddit has a weekly RAW editing competition. Someone provides a RAW file each week, a winner is chosen by the end of the week. It can be a great way to get interesting files to work with, and constructive criticism!
Check it out here: http://redditphotography.com/reddit/raw