500 kHz Beacon Transmitter by GØMRF
This beacon transmitter can frequently be heard on 503.700kHz around the southern UK. It generates a sequence of outputs in 3 different modes The RF electronics in the transmitter produces 6 Watts output but the antenna only radiates 2.5mW ERP.
http://www.g0mrf.com/500_khz_beacon_transmitter.htm

6AG7/807 transmitter for 472 kHz by VK1SV
No commercial radio amateur gear currently supports 472 kHz for transmitting, so a transmitter has to be built from scratch. I selected to build a transmitter using valves.
http://people.physics.anu.edu.au/~dxt103/valves/807-472kc/

A 100 Watt CW Transmitter for the 500 kHz Band by N4ICK
The output of a standard "can" Xtal oscillator at 5,068 kHz is fed to a 7490 divide-by-10 stage. The resulting 506.8 kHz square wave is fed directly to ½ of the 4427 driver, and also inverted by ¼ of the 7400 that feeds the other half of the 4427 (Q and not Q).
http://www.500kc.com/N4ICK/index.htm

A 137 & 500 kHz Class-D Kilowatt by W1VD
The 137 & 500 kHz Class-D Kilowatt transmitter described here was built for use at WD2XNS (136 - 138 kHz) and WE2XGR/2 (505 - 515 kHz) to transmit modes that don't require linear amplification - cw, QRSS and various FSK modes.
http://www.w1vd.com/137-500-500WTX.html

A VXO controlled transmitter for the UK 500kHz Experiment by G3YMC
This VXO transmitter was built fairly quickly once I had got my NOV for operation on 500kHz. It is largely based on the one I built for the UK 5MHz experimental allocation, in fact the VXO and PA are virtually identical.
http://www.davesergeant.com/mf/tx.htm

JUMA TX500 500 kHz Transmitter by OH2NLT & OH7SV
JUMA TX500 is a digitally controlled CW transmitter for the 500 kHz amateur radio band (600 m). The transmitter is available as a component kit which inludes all the electrical and mechanical components.
http://www.jumaradio.com/juma-tx500/

Simple Low Cost MF CW TX by GW3UEP
Low-cost, high efficiency, simple repeatable design.
http://www.gw3uep.ukfsn.org/25W_QTX/QTX.htm