I'm unable to solve this one, happens even on a clean checkout directory. Have run npm install followed by npm start:
Running "checkout" task
Cloning git://github.com/openlayers/ol3.git into .grunt/openlayers-website/repo
Fetching from origin
Fetching tags from origin
Cleaning
Checking out origin/master
Running "install" task
Running "buildpy:examples" (buildpy) task
>> 2014-10-10 16:38:55,477 build/ol-debug.js: node tasks/build.js config/ol-debug.json build/ol-debug.js
>> info
>>
>> ol
>> Parsing dependencies
>> info
>> ol No compile options found. Concatenating 333 sources
>> ERR!
>> Trouble concatenating sources. EMFILE, open '/Users/bartvandeneijnden/opengeo/git/openlayers.github.io_bartvde/.grunt/openlayers-website/repo/src/ol/format/igcformat.js'
>>
>> module.js:340
>> throw err;
>> ^
>> Error: Cannot find module 'rimraf'
>> at Function.Module._resolveFilename (module.js:338:15)
>> at Function.Module._load (module.js:280:25)
>> at Module.require (module.js:364:17)
>> at require (module.js:380:17)
>> at cleanupDirs (/Users/bartvandeneijnden/opengeo/git/openlayers.github.io_bartvde/.grunt/openlayers-website/repo/node_modules/temp/lib/temp.js:96:20)
>> at process.cleanup (/Users/bartvandeneijnden/opengeo/git/openlayers.github.io_bartvde/.grunt/openlayers-website/repo/node_modules/temp/lib/temp.js:119:19)
>> at process.EventEmitter.emit (events.js:95:17)
>> at process.exit (node.js:707:17)
>> at module.exports (/Users/bartvandeneijnden/opengeo/git/openlayers.github.io_bartvde/.grunt/openlayers-website/repo/tasks/build.js:266:15)
>> at /Users/bartvandeneijnden/opengeo/git/openlayers.github.io_bartvde/.grunt/openlayers-website/repo/node_modules/async/lib/async.js:485:30
>> 2014-10-10 16:38:59,287 build/ol-debug.js: clean
>> 2014-10-10 16:38:59,288 pake: build/ol-debug.js: Command '['node', 'tasks/build.js', 'config/ol-debug.json', 'build/ol-debug.js']' returned non-zero exit status 8
Warning: build.py failure: 1 Use --force to continue.
Aborted due to warnings.
npm ERR! weird error 3
npm ERR! not ok code 0