The free software movement has succeeded in creating entirely free operating systems, from bootloaders to kernels to web browsers, and everything in between. But most modern computers still contain significant quantities of non-free code, often hidden in places that users can't easily find them. This non-free code is often used in security-sensitive areas, potentially allowing backdoors to be built into systems without the owner's knowledge.
Where is this code, what does it do, and how dangerous can it be? And, more importantly, what can we do to replace it?