http://all-free-download.com/free-icon/

http://www.iconsdb.com

http://www.photopin.com – finds creative commons on flickr and elsewhere

http://www.stockvault.com

http://www.freepik.com – should be careful with this site, but it looks like it’s legit

http://www.pixabay.com

http://www.picjumbo.com

https://isorepublic.com/