Three-floor market for printers, computers, digital cameras, memory sticks, replacement batteries and all computer and camera accessories. If you're coming to look for brand-names cheap, then look elsewhere as import duties are high. But Chinese products are inexpensive and you can pick up memory sticks, additional camera batteries, external hard-drives and the like very inexpensively.
1 Huaihai Zhong Lu, 淮海中路1号,近西藏南路
Say it in Chinese: "hwai hai zhong loo, ee how".
There's a metro stop at Dashijie station, line 8.