None. Zip. Zero. Not a single one has been independently verified to work. Don't waste your money. Please, please, please don't waste your money for a $5 resistor that makes the car run lean.
If you want better MPG:
don't have roof racks on unless you need them.
Bump up tire PSI a few pounds above the door placard.
Make sure the ATF, and both differentials have good fluid in them.
Make sure car is running well and fully tuned up.
Slow down on interstate.
Unlike VW's and other brands there is no "Chip" for Subarus. The only way to change the programming in the ECU is to get a Tune and reflash the existing ECU. There is an off the shelf brand by Cobb that you do the reflashing yourself but they really only have that for the turbo versions.
Other than a good tune up and driving easy, with a 99 there isn't too much you can do MPG wise. When I had my 99RS I did notice more power and a slight increase in MPG when I put in a high flow cat converter but then you risk setting off the CEL (which there is a way to mask it. Doing the Aerosol Seafoam treatment might not be a bad idea either.
Train yourself to look farther down the road and slow down to increase the chances a light will change and traffic will start moving. Keep the car moving smoothly. Drive as if there is a raw egg on top of the accelerator pedal that must not be broken.(and brake pedal for that matter)
remove ALL unneeded weight, floor mats, maybe rear headrests, - perhaps even to the point of removing seats if you literally would never use them. If you feel you would never change a flat tire and would just call for flatbed tow or for help from a spouse or w'ever that could bring the spare and tools to you - dump the spare and the jack and tools.
next time you get tires, maybe get some that are a few percent larger in diameter. Of course, the speedometer and odometer will be incorrect.