What does an ALK do?

ALK stands for Anti-Lift Kit and is a common aftermarket suspension modification. It's basically a new set of lower suspension mounts that adjust the front geometry and alter the anti-lift character of the suspension. Cost of a kit for my 2002 Impreza WRX is around a few hundred bucks. If you're serious about track work and altering the front suspension you'd be better to go straight for a full kit with new strut tops. For the odd weekend squirt the ALK is ideal. Cheap and effective.

A common misconception is that the kit will REDUCE front end lift on acceleration. This idea comes obviously from the name anti-lift, however the revese is in fact true.

Normal suspension in any car has a characteristic known as anti-lift, which defines how much forward or backward force is transferred into upward and downward force when braking or accelerating. This force results in pitching and diving. A suspension setup with a 100% anti-lift will not pitch or dive at all, but instead transfer all forward and backware force to the tyres. This is bad however, because the tyres have to do much more work and you will much sooner overcome the level of grip they provide. The tyres will also spend less time in contact with the road if the surface is anything but billiard table smooth.

A car with less anti-lift (or more tendency to lift) will pitch and dive more, however the level of grip is higher because the fore-aft load on the tyres is reduced and they spend more time on the road. This is the reason why cars have springs and shocks. Less suspension movement and firmer suspension is good in terms of responsive handling and quickly changing direction, but bad in terms of outright grip, so some sort of happy compromise is required. That's why any car, even F1 cars, have some form of suspension to allow vertical movement, and do not simply bolt the wheels direct to the chassis.

The WRX ALK does two things.
1) reduce the anti-lift characteristic of the suspension, which in turn increases the level of front end grip for the reasons outlined above.
2) increases the amount of caster which in turn gives more dynamic camber when cornering.

In concert with the kit, a slightly different set of wheel alignment parameters can be used, and are generally provided by the manufacturer of the kit.

On the road, the difference is quite noticable. The WRX turns into corners much more eagerly, and higher cornering speeds can be attained. The natural tendency to understeer is reduced and power can be applied to better effect. Overall I'd recommend this modification to anyone.
For the dollars it's a very effective modification.