Basically any thing with a circuit board needs to be treated as hazardous waste. Yet, often they aren't. e.g. "There's a lot of stuff in a circuit board that you really don't want to ingest. Lead, mercury, cadmium and beryllium all have been shown to have harmful effects on humans." And that's not to mention the plastics in the case, etc.
In some cases people just toss electronics with the rest of their trash. Those nasty chemicals then end up in the landfill, eventually polluting the water supplies. In other cases electronics enter a different "waste stream" and often are sent overseas, to countries with less stringent environmental protection laws, where they are dismantled in unsafe conditions via low-tech means. The dismantlers are directly exposed to all those nasty chemicals, detrimentally affecting their health.
Especially bad are old style CRT monitors and CRT televisions. They used a lot of lead in the picture tube to protect the viewers from stray X-Ray's. Yes, lead.
In short, putting electronics out with the trash is a really bad idea. Electronics needs to be disposed of properly, through organizations that know how to properly dispose of the contents. One wishes the U.S. had laws like they have in Europe that require each manufacturer to be responsible for the system which will dispose of the products they sell.
This article: Trash that PC in an eco-friendly way covers a fine overview and pointer to several useful resources.