Many people confuse ramjets with pulsejets and are unaware that although both are quite simple and somewhat similar in design, they vary significantly in the way they operate and the tasks for which they are suitable.
Ramjets have no moving parts -- much like a valveless pulsejet but they operate with cotinuous combustion rather than the series of explosions that give a pulsejet its characteristic noise.
On the face of it a ramjet would seem to be the ideal kind of jet -- no spinning turbines, no vibrating valves and a seemingly ultra-simple construction.
Unfortunately there are two things that work against the ramjet:
Can Ramjets Be Home-built?
Unless you're going to be using it at a minimum speed of 400 mph it will be nothing but a paperweight.
There are some companies that offer to sell you the plans for a ramjet and a helicopter which is powered by these jets affixed to the tips of the rotor blades.
Many people have bought these plans, few have built the engines or helicopters and, to the best of my knowledge, nobody has ever reliably flown one.
What is ignored by such designs are the problems of fuel-efficiency and the fact that the rotorblades of such a helicopter would have to be spun up to almost full RPMs before the ramjet would even operate. This would require significant power to "spin-up" the rotor system and add to the cost and complexity of such a project.
So, I'd advise readers that such projects are an interesting curiosity -- but don't expect that you will end up with a jet-powered craft capable of flying safely or reliably.
So Where Are Ramjets Actually Used?
Many of the low-orbit reusable space vehicles are now considering the use of scramjets for their power-plant while still in the earth's atmosphere but even scramjets remain largely the domain of the drawing board.
Scramjets differ from ramjets insomuch as the airflow inside the engine remains supersonic -- whereas conventional ramjets slow the incoming air down to a much lower speed so that its pressure is increased and it is easier to burn regular fuels.
Here are some links that provide pictures, diagrams and more explanation about the ramjet and its close relatives: