Over in the challenges thread, I discovered that if you make a really light ship, and attach 700 decouplers, they propel it pretty high.
It works because decouplers exert a fixed amount of force, which, when multiplied by 700 decouplers and applied to just one capsule and a bunch of virtually weightless struts, makes you go very, very fast.
There are some problems with the approach: Firstly, 700 decouplers takes around half an hour to load, and you need every one of those decouplers to get your speed. That's because you have to use the heavy manned capsule: you're going to lose 95% of your speed to low-altitude drag, so the low-drag capsule is a necessity over the much lighter probe core. You also can't attach anything to it: it turns out that once you get into the neighborhood of thousand-G impulses, things start to tear off. While attempting to solve these problems, I ended up creating the vessel I dubbed Plaid.
Plaid is a mere 300 parts, loading in seconds. It's basically a line of struts with decouplers stuck on the bottom, and a detatchable probe core. The RCS block is instantly annihilated by the acceleration, but it looks pretty at launch.
Now, I'm sure you're eager to get to the whole 'Mun in 13 seconds' thing I mentioned in the title. This is what happens when you "launch" Plaid.
The game's readouts have a bit of trouble catching up to the absurd speeds generated, but take a look at the numbers in that second screenshot. Plaid is leaving Kerbin at nearly a million meters per second. This is possible because Plaid is built with a strut as its core piece: the probe core is left behind on the ground, meaning all of the decoupler force transfers into those virtually weightless struts, accelerating them to completely insane speeds.
But Plaid isn't just shooting off into space.
Plaid has a destination.
It's going there very, very fast.
So there you have it. A 300 part craft can accelerate to over one million meters per second, and reach the Mun before someone five kilometers from the launchpad hears it leave.
I've created two albums, one showing Plaid's journey, and one showing the construction. The basic principle can be scaled up as far as your computer can handle. Exceeding the speed of light is left as an exercise for the reader.
UPDATE: Youtube user Stun_gravy has used decouplers and ladder physics to launch Kerbals. To Duna. In just 35 minutes. AWESOME!