I own e-stim power boxes which are more powerful and sophisticated like the E-Stim System 2b. But the ErosTek ET232 is still my favorite. It was the first power box I bought after getting hooked on e-stim after a weekend with YngMstrDetriot.
Despite smaller power boxes being available the ET232 is still compact enough to be a travelling companion. With 15 programs (from which I actually only use 5) firing on two possible channels it is plenty of fun in a small box. Being an analogue power box the stimulation is a bit smoother and a bit finer adjustable than digital power boxes. The most important aspect which makes this box one of my favorite toy is how easy it is to use: With just four rotary knows it takes only a minute to explain the controls to a top and then surrender to him.
The ErosTek ET232 costs $299 at Mr S Leather. If you are living in Europe I recommend ordering it from Regulation £335 due to possible custom problems when directly order in the USA. If you want to learn more about my favorite e-stim power box, read my in depth review here. If you are interested in e-stim in general, read my e-stim 101 here.