There are various tuning boxes out there so I’ll not cover them all. Pretty much all tuning boxes work by fooling various sensors on the engine to make the ECU inject more fuel. If you’ve got a 2/4/600 then the one I recommend is the TU1 from Rover Ron. It’s cheap enough and does the job well. I wish I was on commission from him due to the number of people that I recommend them to but I’m not. He’s just a bloke who has developed it and provides excellent service.
Sadly since writing this Ron has discontinued this product and has replaced it with a new more expensive product. I hope this one is as good as his last. I’ve not tried one yet though although he has incorporated a couple of ideas I gave him I believe to remove the unsteady idle that can occur with the TU1 when it is turned up too high. Pay a visit to his site even if you aren’t looking at buying his stuff as there is lots of useful info on there.
If you’ve got a 25/45/etc then it is usually easier and cheaper to get a remap than fit a tuning box.
(I’m not including MAF amplifiers (MAFAM’s) here as I don’t really count them as a tuning box, although the can be used to fool the ECU into injecting more fuel than it normally would).