I bought a pair of 3" grenade balls from elitefts.net. I think they'll be a lot of fun as a grip tool and just to change things up a bit from time to time. I think they were on special for about $22 each + shipping when I ordered them, but they are a little more now. I've known people that have rigged up grip tools using baseballs and softballs, and that's essentially what these are (but made of metal).
What exercises will I be doing with them? Tonight, I did some light deadlifts with the grenade ball(s) connected by a small chain to kettlebells. I think they will also be perfect for light reverse and hammer curls, light rows, shoulder raises, and bent-over laterals. Pull-ups will be a challenge with these, but I'll give them a try soon.