Been using the Dry & Dry brand PET Blue packs for a few years, and I think they're great. They're rechargeable by defrost nuking or in the oven (I use the oven at 200 degrees for 2 hours). The packets are clear plastic, and when they turn pink they're full. A couple hours in the oven and they're dark blue again.
I use them in all my gun cases, such as a rifle case where I'll toss 3 packets each in them. They'll work for about 4 months before needing to be recharged. Just some assistance to beat the rust. Very cool.