So what is it these things do??? Is it just the placebo effect? Where does it install? I've seen them all over eBay for years.

 

Usually installs in the IAT (intake air temp) sensor. Tricks the IAT into thinking it's a certain temp all the time like 55 degrees. So when it is colder than that it runs lean, when it is warmer it runs rich. No validity to even the thinking behind doing it, just easy money for them.