I've gone through a couple of step-down transformers (220v → 110v) over the course of my travels. Each one I've purchased doesn't quite work as advertised; usually, the output voltage is still too much, and I end up frying one of my electronic devices!
For instance, during a trip to Shanghai last year, my traveling companion had a hair straightener plugged into a step-down converter (actually, very similar to if not the same model as the one HaLaBi suggested in the comments below). The hair straightener overheated and the heating elements melted through the plastic housing. It was not a pretty sight.
What step-down transformers are available for travelers that reliably convert 220v to 110v without spikes... in other words, that actually do what they're supposed to do!
Bonus points for compact design (fits on a power strip) and has adapters for different plug styles.