I haven't had this baby through a full dry season yet, so I may have to edit this if it doesn't work, but it seems pretty effective. It's basically just a sponge inside a case that fits right under the strings in the soundhole. The best things about it are that it's easy to insert and remove and because of its positioning it won't move or spill or bang into anything even if the guitar is moved around.
Price: $15.00 (new)
Where Obtained: Musician's Friend
Easy to insert and remove in the soundhole. Easy to moisten. Unlike some other humidifiers, this one will not move around if the guitar is moved.
It looks durable, but I've only had it a little while, so I can't comment.