Textures and HDRI

cgtextures - several great free ones and membership offers really high resolution options.

openfootage - great source for some free hdr's, timelapse footage, and textures.

Aversis - HDRI maps - some free and purchasable ones

Mayang's Textures - Free textures

Texture Warehouse -  Free textures

Texture King - Free textures

Texturise - Free textures

TexureZ - Free textures

Stock and Reference Images

Stock.xchng -  great source for some free stock photography

3D.sk -  Human photo reference site

DeviantArt Resouces-  deviantart members have put up several great stock and reference images to use

Materials

Vray Materials - Great free vray materials

MR Materials - Mental Ray Materials

Patterns

Pattern 8-  Great free repeatable pattern site for graphic design

Art Deco Patterns- Art deco patterns for reference and ideas

Architectural Resources 

Manufactures of real materials have great images of there products to use in renderings below are links to those sample library images.

Wilsonart Laminates

Formica Laminates

Laminart Laminates

Nevamar Laminates

Pionite Laminates

Corian Solid Surface

Wilsonart Solid Surface

3-form - Decorative Resin Architecture panels

Veritas - Decorative Resin Architecture panels

Architectural Elements are important to every environment.  These elements gives the area a character and style plus helps to tie areas together.  You can take a same environment layout and just by changing a few of these architectural elements can you have several different concepts using the same layout.  So having a diverse knowledge of different kinds of architectural elements will open more options to creating new unique environments.  

I have always been in love with architecture and man made objects.  I am always searching books stores for new architectural books and many are in the reduced price section.  Don't over look these areas there are some great gems in there.  There are a few that I have found to be great and I use often.

Must have books and I got them all for under 7 dollars and some even 2 to 3 dollars with a sale from Barnes and Noble are below:

Pictorial Encyclopedia Historic Architectural Plans, Details and Elements
by John Theodore Haneman

376 Decorative Allover Patterns from Historic Tileworks and Textiles
by Charles Cahier, Arthur Martin

Ornamental Ironwork: Over 670 Illustrations
by A. Durenne

Celtic charted designs
by Co Spinhoven

A Victorian Housebuilder's Guide
by George E. Woodward, Edward G. Thompson

Treasury of Art Nouveau Design & Ornament
Dover Publications

Traditional Chinese Designs
Dover Publications

Other books I use for architectural element I have in my library but are more pricey