Frictional Games Forum (read-only)

http://www.cgtextures.com/ - Probably the best free source, requires an account and has a limit to the amount you can download per day.

http://lostandtaken.com/gallery - Totally free, no limit on download.

http://tileabl.es/packs - Totally free, high quality, not that many textures though.

Reply below with more resources so the list can grow!

I used sharecg.com to have some textures, not a great resource though.