npm list for local packages or npm list -g for globally installed packages.

You can find the version of a specific package by passing its name as an argument. For example, npm list grunt will result in:

projectName@projectVersion /path/to/project/folder
└── grunt@0.4.1

Alternatively, you can just run npm list without passing a package name as an argument to see the versions of all your packages:

├─┬ cli-color@0.1.6 
│ └── es5-ext@0.7.1 
├── coffee-script@1.3.3 
├── less@1.3.0 
├─┬ sentry@0.1.2 
│ ├── file@0.2.1 
│ └── underscore@1.3.3 
└── uglify-js@1.2.6

You can also add --depth=0 argument to list installed packages without their dependencies.