Another reprint:
Ok, although the precise reply completely
depends on your chosen sport, the actual weakness you wish to strengthen
and so on the following might be of some use.
Grippers
These
train crushing grip. Adapting how you use them can help mimic some
holds used in sports and if nothing else you'll have a hell of a
handshake. 
Wrist roller
A
very simple device which can be used to help law enforcement officers
(Correctional, Police, doormen and so on) and those involved in hand to
hand fighting (wrestlers, MMA, etc) in controlling subjects. Because the
two inch thick models are on a par with a thicker wrist working hard on
this will help give you greater control when you've got someone pinned
and am looking for a 'lock'. Adding a little padding (say using the tape
used on some handles) will give the 'give' that flesh over bone has and
this make it even closer to what you do.
It's also a
very simple item and easy to make (although you'll love the heavier
knurled versions if you take it seriously) which can be truly brutal
when worked hard. Bone deep muscle burns, hands sore... you'll know
you've worked the grip AND the forearm.
Thick bars
Like
the wrist roller a good way to mimic real world work. From forearm
curls (much harder than a standard bar), to press work (sits better in
the hands) to picking it up (deadlifts etc) all are made hard when you
use a thick bar but again those working with prisoners and the like will
find handling a fighting limb much easier.
Small block pinching
One
for the climbers. Think about some of the climbing walls you've seen
and the holds they have to grip onto. Now in the case of the free
climbers (no rope) you MUST hold on and you MUST have a grip. But if it
goes down you come.... ouch. So you can find the guys doing low altitude
(6-8 feet from the floor) hangs on beams and so on. Quite often done
for time. Providing the block, pin, etc isn't dropped on your toes it's a
far safer way to work that grip required.
