I use the WD My Cloud, it connects directly to my router and is external, has its own power supply too so doesn't need USB power. There is software you can install on your phone/tablet etc which will allow you to access the file to watch etc without your PC been switched on. I got mine from Maplin for £50 for the 1TB. It also runs DLNA so will connect to a DVD/Blu ray player and TV boxes like apple TV, WDTV box etc.
Great kit, well supported and works like a charm. Easy to upload files via the software or network connection via your PC and you can access it via the web too if you set it up correctly.