These days, one can attach all kinds of devices to computers, both desktop and embedded, mostly along the lines of USB-attatched gadgetry. So I'm thinking a device to monitor air quality.
Indoor air quality is mostly threatened by a few particular chemicals, which we can develop probes for. Namely: Carbon dioxide, carbon monoxide, a few volatile organic chemicals, radon, mold spores, Legonella bacteria, and ozone. Probes for detecting these chemicals will be attached to a small embedded computer, capable of reporting in to a desktop machine via wired or wireless network. Or, if the level of any particular chemical gets beyond a certain threshold, the device would sound an alarm.
People wanting to improve their indoor air quality should invest in filtering systems, increase ventilation, or plant plants. Many people are greatly surprised at a house plant's ability to absorb airborne pollution, partially because the plant typically gets some symbiotic help from friendly bacteria.
BREAKING UPDATE: Xinxin Wang and Zhi Chai have beaten me to it. Excellent work, designers.