This $3.75 gadget is extremely handy for custom low foot print server builds like FreeNAS or ESXi. You install your small foot print OS onto a USB stick and hide it away. In fact, FreeNAS recommends installing onto a flash drive. Instead of having a USB pen drive sticking out the back of you server, you can simply hide it away inside the chassis like this:
Here, I have it in my Fujitsu MX 130 taped to the side of the drive caddy. I can free up the SATA ports for drives and my OS boots from the USB stick. Cool indeed.
Other uses included setting up an internal usb wifi. This is great for those who hackintosh and need to use a specific wifi dongle.
Link: Amazon StarTech USB A to USB Motherboard 4-Pin Header F/F 2.0 Cable