If you go on YouTube, you will find a large number of machines that work for almost forever. But why do all of them stop working after some time? Which Law (Other than Conservation of Energy) prevents a machine from running till eternity?
Most don't work due to the laws of thermodynamics, usually the first law which states that energy can neither be created or destroyed as you mentioned in your question . This stops the opportunity for many perpetual motion machines as most lose heat (and therefore energy) in the process. Without putting more energy in the total energy of the system will run out.
Aside from conservation of energy there is the second law of thermodynamics, which states that the entropy of any isolated system always increases (entropy is defined as the measure of a system's thermal energy per unit of temperature that is unavailable for doing useful work). This means that over time less of the systems energy will be able to do the work, and the machine will eventually stop.
The ones that contain magnets and objects rolling up ramps, dropping down and rolling up again etc. don't work because eventually the magnets will stop being magnetised.
Other's I've heard of are things such as capillary action by water going up a tube and dropping down. The problem with ones such as that is that if the water's gone up against gravity, it's not going to drop out without external force.
Hope this answers your question - commennt other supposedly perpetual motion machines if you're not sure of their problems and I'll see if i can find one :)
Which Law (Other than Conservation of Energy) prevents a machine from running till eternity
Why is the law of energy conservation not fulfulling for you to answer that? There is always energy conservation, and that is the sole reason that you can't draw more energy out of a machine than is put in.
That the output is always even less than that is a result of efficiency considerations (the Carnot limit e.g.)
So, drawing energy from a machine will cause it to eventually stop. But if you don't draw energy from a motion, then theoretically it will continue running forever. A rolling wheel will in theory never stop rolling if nothing stops it. A space ship drifting in space will never stop drifting but forever have constant velocity, until something stops it. But here on Earth we never see anything lasting forever. There is always friction, deformations in materials, air resistances etc. even in the smoothest gears, and all that cause small energy losses. Nothing is ideal; everything will stop eventually.