OS X Yosemite 10.10 bootable USB (DMG File) (14A389)
Use the "Restore" option in Disk Utility to write it down on a USB-stick.

Enjoy!

How to do info..

Format Your USB Flash Drive

1. Insert the USB flash drive into your Mac’s USB port.
2. Launch Disk Utility, located at /Applications/Utilities/.
3. In the list of drives attached to your Mac, select the USB flash drive device.   In my case, it’s called 14.9 GB SanDisk Cruzer Media.
4. Click the ‘Partition’ tab.
5. Select ‘1 Partition’ from the Volume Scheme dropdown menu.
6. Enter a descriptive name for your flash drive; I chose "USB Install Drive"
7. Select Mac OS X Extended (Journaled) from the Format dropdown menu.
8. Click the ‘Options’ button.
9. Select ‘GUID Partition Table’ from the list of available partition schemes.
10. Click ‘OK and click "Apply"

Now the USB flash drive is prepared you can Restore "OS X Base System.dmg" to your flash drive.

1. Click on "Restore" in the Disk Utility window.
2. Drag "OS X Base System.dmg" to "Source"
3. Drag the usb flash drive (in my case "USB Install Drive") to "Destination"
4. Click "Restore" en enter your system password.
5. Done