stock.xchng is a free stock photo site. Most of the artwork has a standard licence that allows them to be used for most purposes free, but not for pornographic or unlawful purpose, for spreading hate, etc. Really useful and I've been grabbing a lot of their images for use in roleplaying.
Block Posters allows you to upload an image and it will turn it into a block poster of whatever size you want, split into individual A4 or Letter pages, as a pdf document. I've done a few that I'm intending to print out and put on walls.
Genius Printing is a canvas print shop based in Melbourne. They look pretty good but I haven't tried them out yet.
The Peirce Clipart Collection is useful, and it's free!
Free vector graphics at Vecteezy and FreeVectors.net.
Lots of free icons as IconArchive.