Indispensable for powering up those new USB 3.0 add-in cards that require motherboard power for the ports. Most new machines have SATA-style power plugs coming from the power supply, while the USB card has either 4-pin Molex or floppy connectors. Get one of these if you have the latter, and your new USB card is up and running with power to all ports.
After purchasing a 4Tb ext drive that can run via 3.0 USB connection, I found I needed a small board for my desktop which needed a connection to the power supply. (My existing desktop only ran USB 2.0). My only available connections were SATA and the new board required a small female Molex connector for the power supply connection. I could not find what I needed locally, or on-line. A very knowlegeable computer expert at the Dell Community website answered my search with a link to this product on monoprice! I ordered it and it arrived promptly. It was easy to install and worked perfectly. Thanks to monoprice for carrying this product and delivering it in a reasonable amount of time!
I bought a Multimedia card read from store [x] and when it came it only had the older 4 pin power connector. My PC only had SATA type power connectors. This cable fixed the problem. I looked everywhere and finally found it where you can find every cable in the world... at monoprice. Thankfully monoprice has drastically improved their website's search capability.
Good SATA Power to Molex + Mini-Molex Power Adaptor
I just built a new computer. My new power supply did not provide any Mini-Molex (Berg Plug, 3.5" plug) power plugs. However, I had an add-on PCIe card that needed this type of power plug. I also needed the Molex plug. This adaptor solved both problems at once. It works fine.