I personally have been doing a lot of gripwork since 2005, started off because I wanted to get my grip stronger for the sport of strongman but now I enjoy doing all sorts of different things for it. There's a few exercises I enjoy immensely, but the one exercise and training tool I think is invaluable for karateka is block weight tossing.
To make a block weight, all you have to do is cut off one head of a hex-headed dumbbell. (The bonus of this is now you can use the other part that has the handle and extra head as a chi ishi). A 25-30 pound dumbbell is a good start, that should give you a 12.5 to 15 pound block weight. If you have big hands, you can try a bit heavier, but you definitely want to start out light as it's harder than it looks.
The actual exercise itself is easy to learn but will give you a great workout. You simply stand normally, holding the block weight palm down. You toss it up in the air, then catch it with the other hand palms down, then toss it back and keep going for as long as you can.
Here's a video:
YouTube - Block weight juggling
Training with block weights will not only strengthen all your fingers (including the thumbs, which are often neglected in many grip exercises) but it trains hand-eye coordination and explosiveness (you need to close your hand quickly around it, which IMO can help punching power). You can do a lot of different things with it. I once took my weight (I have a 40# head, so it's about 20 pounds) for a walk up and down my street, just tossing it back and forth to myself the whole time. It was tough but fun.
If you're serious about grip training, it's well worth the investment.