title_body: Are there any free open-source custom firmwares for e-ink devices? I'm curious if there are custom firmwares for e-ink devices similar to what Rockbox (website, wikipedia) is for digital audio players.
upvoted_answer: It depends on specific device and its architecture.
  
  Those that are built on top of Android, typically tend to get AOSP ports, e.g. Nook (here's Simple Touch instructions), or Kindles - the latter even has CyanogenMod ROM. 
  
  Sony PRST 1 was hacked to expose full underlying rooted Android but I'm not sure if AOSP based ROM is available for it - this might be it.
  
  I'm not sure if there are any non-Android-based mainstream eInk readers - most of the ones I heard of (which is not a lot) seem to be Android under the covers and thus can be opened up (e.g. see Kobe Glo).