One of my favorite curl variations is the Zottman Curl. It is basically a regular dumbbell curl, but at the top you turn your palms down and lower the weight in a reverse curl. Since the reverse curl gets at your forearm strength more, and is generally less strong than a regular curl, you are basically overloading the eccentric (here, the lowering) portion of the exercise.
You can see Joe DeFranco demonstrating these in the first 7 reps of this 21-rep set:
21's ver. 2.0
Curl up, turn, lower down. Elbows stay close to your sides and the weight is lowered slowly to ensure you're not just dropping the weight down and catching it at the bottom.
These can be done double - as seen in that video, alternating (left then right, repeat), or one arm (all reps on one arm, all reps on the other arm.)
Tabletop Zottman Curls
This variation is one I first saw in this video by grip strength competitor Jedd Johnson.
Tabletop Zottman Curls
Like preacher curls, this has the effect of stabilizing the upper arm, preventing any "cheating" by swinging the body. The dead stop/dead start for each rep also means you can't use any momentum. Every rep will be as hard as the previous rep, and require a strong grip to squeeze the dumbbell off the bench and return it gently at the end of the movement.
Circular Zottman Curls
I've yet to find a video, however, showing this version of the exercise. It is described John Little in the book "The Art of Expressing the Human Body," which discusses Bruce Lee's workout routines and physical training methods. Bruce Lee was apparently a big fan of the Zottman curl, but it's not 100% clear if he did the "normal" version or this version, below.
"Curl the dumbbell in the left hand to the left shoulder, keeping the upper arm still but permitting the dumbbell to pass toward the right side of the body during the movement. When the elbow is fully bent, rotate the hand so that the palm is downward; then lower the dumbbell to the starting postion, at the same time taking the dumbbell away from the body as far to the left as possible (without altering the position of the left upper arm). When the left hand has been rotated and the weight is being lowered, the dumbbell in the right hand should be curled (across the body) to the right shoulder. [. . . ] Each dumbbell makes a circular movement, which should be performed smoothly and rhythmically."
That is a very different motion, combining a cross-body motion similar to a standing concentration curl with an external lowering of the weight. I've never seen this demonstrated. It makes for a somewhat different feel to the curls, too, although it's not clear that it does anything more to increase your strength or muscle size compared to a "normal" Zottman curl.
If you're finding your grip strength and forearm strength is lagging, this is a great way to address them (even more so with thick handled dumbbells, Fat Gripz, or Tyler Grips.) It's a good way to get both extra biceps work in while training your forearms and grip, too.