I've built many HTPCs in my lifetime and here's one that's kinda cool. Take the Gigabyte H55N-USB3
motherboard, which measures 17cm x 17cm, add an Intel I5 processor, ram, SSD, and a mini power supply inside the motherboard box itself and run your HD movies in a very tiny small form factor.
My current setup has a Ceton InfiniTV 4
so I'd definitely need to put in a case for the PCI-E tuner card but hey, if you don't care about that and just want to run a Blu-Ray movie or stream media, this is a cool project that shows you how it can be done.