Pretty straightforward: Google image search has this nice UI mechanic that pops a larger-scale image with info on mouseover.
Lookbook does this too.
What would be the easiest way to replicate this effect, and what tools should I look for?
here's a sample i just did in 30 mins similar to my old plugin
it should be enough to get you started i hope. there are certain things i botched due to time
I would bet they're using JQuery. A lot of Google effects use this.