The below list is incomplete.
The most up-to-date info is in https://github.com/imageio/imageio/graphs/contributors

- Almar Klein
  Plugin system
  Most plugins so far

- Anthony Tanbakuchi
  Many improvements to the ffmpeg plugin

- Anselm Levskaya
  Fixed bug (causing segfaults) related to freeimiage GIF writing

- Arash Rai
  Animated GIF looping

- Chris Dusold
  Improvements to ffmpeg plugin

- Felix Kohlgrüber
  FFMPEG improvements

- Ghis Vaillant
  Debian packaging

- jackwalker64
  Fixes for ffmpeg plugin

- Joel Nises
  Improvements and fixes for the FreeImage plugin

- Juan Nunez-Iglesias
  FEI plugin

- legionaryu
  Fixes for the GIF plugin

- Marcin Konowalczyk
  added `set_image_index()` to Reader object

- Mark Harfouche
  Speedups to pillow and ffpmeg by avoiding memory copies and conversions.

- Matt McCormick
  Improvements to simpleITK plugin

- Niklas Rosenstein
  Fixes for using Pillow plugin in threads

- OrganicIrradiation
  Added support for 16 bit PNG's

- Paul Müller
  Fixes for frozen applications
  Improvements to download mechanisms and entry points

- Peter Lee
  Fixes to animated GIF

- Rob Reilink
  AvBin plugin

- Steven Silvester
  Tiffile plugin
  GDal, simpleitk, and fits plugins

- Tim D. Smith
  Improvements to ffmpeg plugin

- Zach Pincus and others from the scikit-image team
  Initial version of freeimage plugin
  Initial code for findlib and lib downloading

- Zulko
  Various contribution related to the ffmpeg plugin.

- Addison Elliott
  Add support for 8-bit paletted BMPs