Here are projects that people in the Toolkit community have been gracious enough to share. They aren't written by us at Shotgun and we can't vouch for them, but we'll definitely answer any questions we can about them. If you have a project you want added to the list, just email us at email@example.com.
Project URL: https://github.com/robblau/tk-katana
Project Contributor: Lightchaser Animation
Project Description: A Shotgun Engine for Foundry's Katana
Project URL: https://github.com/tremolo/tk-modo
Project Contributor: Lutz Pälike and Walking The Dog
Project Description: A Shotgun Engine for Foundry's Modo
Project URL: https://github.com/diegogarciahuerta/tk-natron
Project Contributor: Factor64
Project Maintainer: Diego Garcia Huerta
Project Description: A Shotgun Engine for Natron, a free and open-source node-based software application.
Project URL: https://github.com/basestudio/tk-maya-playblast
Project Contributor: BASE Studio
Project Description: App to publish playblasts from Maya. See https://goo.gl/5oJTv0
Project URL: https://github.com/baitstudio/tk-multi-renderfarm
Project Contributor: Bait Studio
Project Description: App to submit work to the farm. See https://goo.gl/ew6mkD
Project URL: https://github.com/janimation/tk-shotgun-publishrenders
Project Contributor: Janimation Studios
Project Maintainer: Dave Sisk
Project Description: This app searches the directory structure of a project to find existing published files or file sequences, then registers them in Shotgun as published files if the published file objects don't already exist.