I get that taking your own photoreference is awesome, and useful, etc, but the dude was asking for photo reference sites, and I don't think harping on him to go take more photos is going to really meet his needs. Let's not pretend that people don't use cgtextures.com or whatever else at work.
OH nice, I'll be adding a few of these to my list.
I've had some success with flickr but it could take some digging, and people are stingy with what they let you download.
Yes, if you want to get serious, get a more or less decent SLR camera, a prime lenses and go take some photos. Fix any of the perspective errors you might have on the original photos, and voila, you have your texture source.
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Seriously, grab a camera, and go take pictures.
Or, make your own.
http://plaintextures.com/index.php
http://webtreats.mysitemyway.com/category/textures/
http://www.imageafter.com/index.php
http://seamlesstextures.deviantart.com/
http://texturize.deviantart.com/
http://texture-central.deviantart.com/
The studio I work at has done this at times, but only if very specific stuff was needed.
I've had some success with flickr but it could take some digging, and people are stingy with what they let you download.
Here is a new one:
http://lostandtaken.com/
and
http://www.antiques-atlas.com/
Here's a few that havent been posted yet:
http://www.texturelovers.com/
http://www.surrealtex.com/
http://textures.forrest.cz/
http://www.mayang.com/textures/
http://freetextures.3dtotal.com/index.php?t=20100127&p=0&cid=17
http://www.2textured.com/
http://freetextures.3dtotal.com/
http://www.texturearchive.com/
and of course... the fantastic CGTextures.com, best textures DB you can find.