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-