Grunt debugging from Webstorm

You have to run grunt-cli as a Node application:

• Create a new Node.js Run/Debug configuration: Run->Edit configurations...
• In Path to Node choose your node binary, ie: /Users/someuser/nvm/v0.10.5/bin/node
• In Working Directory, choose your Gruntfile.js directory, ie /Projects/someproject
• In Path to Node App JS File, choose your Grunt CLI path (you can choose the grunt-cli symlink created on your node bin directory, WebStorm will use the symlink target), ie: /Users/someuser/nvm/v0.10.5/lib/node_modules/grunt-cli/bin/grunt(C:\Users\someuser\AppData\Roaming\npm\node_modules\grunt-cli\bin\grunt on Windows 7)
• In Application parameters enter the Grunt task to run, eg default or test

Click on Run or Debug and you are done :)

I just happened to try this in WebStorm 10 today. I'm not sure if it's a new feature but WebStorm have integrated the steps described by @diego so you don't have to perform them manually:

1. Click View > Tool Windows > Grunt to display the Grunt window in WebStorm.
2. Right-click on any task and choose "Debug"

You can set breakpoints etc. in Gruntfile.js just like you would any other file.