A $25 Oasis from
http://elderly.com/accessories/items/OGH2.htm. According to them, they've tested it against their analog hygrometers and it is accurate. I've been using it for the past 3 months and my guitar definitely has been kept in the preferred RH zone. It's been sounding great w/ no signs of whatsover of dryness. The heat has been on a lot where I live and the house is definitely very dry, but I use this to measure the RH in my guitar case and it's been good for me.