ISO Republic

http://isorepublic.com/

Fairly new to the scene, but a great list of architectural and other images rich with vivid colors - quite eye-catching.

Hi Res Stock

http://hiresstock.com/

You can use all photos here for your work – themes, templates, apps, background, you even could print them on a t-shirt.

StokPic

http://stokpic.com/

Free Stock Photos For Commercial Use

(Stokpic License) Basically You can do anything apart from redistribute.

Public Domain Archive

http://publicdomainarchive.com/

A nice minimalist design website where the creators scour the web for high quality public domain images and bring them to you!

All the Free Stock

http://allthefreestock.com/

The search engine for free stock images, photos, videos, mockups and fonts. All created under the Creative Commons Zero license and ready to use for personal or commercial websites. No attribution or credit (source) required.