What parts of the RaspberryPi (A,B,B+) are not open-source (yet)? I mean any part of the device which requires some kind of software to run.
GPU requires a non-free binary blob. Moreover, unlike on desktop computers, you cannot avoid using it even if you do not need graphics at all – due to design, Raspberry Pi just would not start without that blob.
Since it’s quite large for firmware: about 2 Mb¹ (in binary form, of course), I doubt, that it would be been legally reversed some day and thus free/libre implementation would be written.
¹ For comparison: non-free blobs required for Radeon HD 8[1-4]00 series are about 60 kb large.
P. S. Actually, when early in 2014 rpi.o announced good news which gave ability to use a free/libre graphics driver (yes, much more non-free software was needed before), in comments Liz Upton said that they were “going to be working on a minimal, open [source replacement for the] blob so you’ll be able to boot without it, but that won’t happen for a while; it’ll be a fair amount of work”. But I have not heard about any progress on this task.