Electronic equipment contains toxic materials:

Computer and television monitors with cathode ray tubes (CRTs) contain four to eight pounds of lead on average. The EPA has identified electronic products as the largest single source of lead in municipal solid waste.

Printed circuit boards in computers, music players, and other electronic gear contain toxic metals such as chromium, nickel, and zinc.

Batteries may contain nickel and cadnium. 
                                                                                                                                                                                                                                                     
Relays, switches, and liquid crystal displays (LCDs) may contain mercury.

Plastics used in many electronic products also contain flame retardants that are toxic and persist in the environment.

Very interesting interactive produced by National Geographic
http://ngm.nationalgeographic.com/2008/01/high-tech-trash/computer-interactive

Hazardous Waste: Electronics, Lead, and Landfills http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=1247531

National Geographic: High-Tech Trash http://ngm.nationalgeographic.com/2008/01/high-tech-trash/carroll-text