Here's a good place for you to share your experience with any piece of training equipment you have used in the past or utilize in the present. I've found a few items that have really helped improve my training results, and I'd love to read about some more. Always looking to improve, right? If you got something you'd like to share, please do! Tools, gear, anything.
To start things off - My personal two favorites at the moment have been the "IronWoody Hand Gripper" and the "PowerLung".
"Iron Woody Handgripper" Hand Grips - Handgrips - Hand Grip - Handgrip from Iron Woody Fitness
Fully customizable handgripper with over 60 settings. I have yet to personally close anything close to the 75% difficulty. The maximum is without a doubt over 500lbs IMO. Two handgrippers on the link, one with a more fancy handle than the other - I own the Woody Gripper, the less comfortable one, and I have had success in throwing a small towel around the grips for the heavier sets.
"PowerLung Sport" PowerLung: Improve breathing for the active lifestyle, including Performance, Competition, Fitness, and General Health with reduced breathlessness, increased lung capacity, improved VO2max from respiratory strength training.
A fully customizable breath trainer. Very effective at developing a strong, deep breath, AND a strong exhalation. Very beneficial on cardio sessions, where you feel like you're not getting enough air. Many breath trainers out there train the inhale, but not the exhale. PowerLung trains both. The recommended start is 10 full breath repetitions for 3 sets, twice a day. A note - this is the most difficult model of their lineup, and you will more than likely need to start off on the minimum settings. Very difficult at first, but then again it's tough to build strong breathing muscles without training them directly. Very effective tool for any sort of cardio activity.