High-density foam roller with molded edges.
Way back in 2004, T Nation introduced meatheads to the foam roller. Since then, it's been used – and sometimes abused – by just about everyone who has ever stepped foot into a gym. Initial claims about foam rolling "breaking up scar tissue" have been dispelled, but it's still great for relieving tightness and speeding recovery after a workout.
These days, the foam roller market is huge. You can buy bumpy ones that look like off-road truck tires and even ones that vibrate. (Your girlfriend likes that one.) And you know what? None of that crap is necessary.
All you need is a firm (but not too firm) roller with a fairly smooth surface. No bumps or ridges or batteries required. This one from AmazonBasics is made from molded polypropylene to maintain shape and firmness with heavy use. We kinda like the red-flecked one because it looks like a crime scene, but other colors are available. – Chris Shugart