There are loads of boost pedals around at the moment, some offering tonal adjustment as well as boost, but on the surface T-Rex's Karma Boost looks as simple as it comes, with just a single knob that can deliver 16dB at full whack.
The pedal, though, offers non-linear boost so that the character of the sound will change at different knob positions.
With the control knob set to minimum, there's no apparent boost at all, but at around a 10 o'clock setting there's a nice, subtle lift that works well, transparently giving a driven amp a bit extra for solos.
Further advancement of the knob gives you more of this, with a pleasing edginess and presence easing in at higher settings.
That extra presence is also there when used with a clean valve amp: the Karma boost won't turn it into a snarling monster, but it will drive it into a useful degree of breakup.
A tasteful booster with the added advantage of a buffer to make up for long cable runs.