The term MEPT is normally used to describe a low power QRP or QRPp (less than one watt RF output) transmitter used to transmit clearly identifiable signals which may be identified by a "grabber".
There are many different types of MEPT in use around the world, but one of the more popular designs is that by Hans Summers G0UPL.
This has been made availabe in kit form at a very attractive price by QRP Labs, and I have built one of these and used it successfully on the 10MHz (30m) band.
Basically the design is a VXO (variable crystal oscillator), buffer and a small MOSFET PA which produces a clean signal of around 100mW. The really nifty bit is the keyer circuitry which comprises an ATMEL AVR microcontroller programmed with the user's callsign and this is used to modulate the transmitter converting it to DFCW (Dual Frequency CW - a kind of slow FSK - Frequency Shift Keying) by "pulling" the oscillator by a few hertz in synchronism with the callsign being read out from the PIC's memory.
Here's my own example as first built looking remarkably similar to the one shown on Hans' web site! The basic unit is powered from a 5.5V DC supply and takes a few tens of mA.
As is usual with this kind of equipment, a whole array of modifications have proliferated, and my version has been adapted to run from a nominal 12V supply, simply to reduce the probability of me inadvertently blowing it up! Mine now resides in a "recycled" aluminium diecast box with an LED flashing (slowly) in sympathy with the keyer. It all works very nicely.
The PCB is just the right size to fit into an "Altoids" ™ tin, and this kit has no doubt contributed to the global sales of that particular confectionery product, although I adopted the alternative construction method described above.