The foam roller!
I had seen these at the gym but for years never knew what exactly they were used for.
When my last gym shut down we were allowed to take some of the extra equipment, these were the FIRST to go, which clued me in to their value.
These are the ideal tool for stretching out after a hard workout. The pressure exerted by the roller gives all the benefits of traditional streching plus helping to break down soft tissue adhesions and scar tissue.
This article gives a detailed explanation of how and why it works as well as photographs of different stretches that can be done with the roller.
A couple months ago I suffered what felt to me like a serious achilles injury. Thankfully it wasn't torn, but it was very painful. I was told to roll out my calves frequently, going from just below the knee to just above the achilles. Crossing one leg over the other I was able to put additional pressure on my muscles. I won't lie, it hurt, but in a good way.
Like any stretching, results aren't immediate, but over time it really helped. Whenever I feel my achilles acting up it's typically because i've neglected the stretches.