This motor speed controller off eBay was a bargain at under $5 with free postage.
It uses a simple 555 timer circuit and has a fuse as well as reverse polarity protection.
I tested it with a 12V motor from a car electric window regulator and could vary the speed from full speed right down to a crawl.
This would be ideal for slowing down windscreen wiper motors to be used for moving Halloween props.
Claimed Specs:
Working Voltage: DC 12V – DC 40V.
Control Power: 10A max.
Static Current:0.02 A ( Standby ).
PWM Duty Cycle:10% -100%.
PWM Frequency:13 KHz.
Size: 60*56*27mm