Also sometimes called Stompboxes these were first built in the 1960's and then developed much more in the 1970's.. For a good description of many makes I recommend you get hold of the book Stompbox by Art Thompson published by Miller Freeman.
Electro Harmonix American firm well liked by guitarists for their unique effect units. now called new sensor corporation .
Digitech make a range of effect processors aimed at both the Guitarist and Studio user.
Roland [also known as Boss ] effect units vary from the small distorsion units,the Studio effect units thru to the large space echo units .
Schaller [ normally known for their Guitar parts] did make one or two Effect units.
Vox also known for their Guitar Amps and organs did make several Effect units, including one of the first Wah Wah units.