Knife-Spork:
This plugin helps you control versioning of your cookbooks and also prevents you from accidently commiting things you don’t want to.
$ gem install knife-spork
https://github.com/jonlives/knife-spork
Knife-ec2:
Obviously this is a connector to ec2. I use Amazon ec2 so i am biased
$ gem install knife-ec2
https://github.com/opscode/knife-ec2/
Knife-preflight:
A preflight plugin for Chef::Knife which lets you see which nodes and roles use a particular cookbook before you upload it.
$ gem install knife-preflight
https://github.com/jonlives/knife-preflight
Knife-env-diff:
A plugin for Chef::Knife which will diff the cookbook versions of two or more environments.
$ gem install knife-env-diff
https://github.com/jgoulah/knife-env-diff
Other mentionables
Knife-slapchop:
Threaded bootstrapping of aws nodes
https://github.com/kryptek/knife-slapchop
Knife-ironfan
Cluster Deployments
https://github.com/infochimps-labs/ironfan
Search for more knife plugins:
$ gem search -r knife-
