Ebay $40 ultra mini geiger meter for android phones.

Works great. Plugs into the headphone jack. It takes a few seconds to get going, not like the almost instant meter on an old school geiger counter. Cool thing is if you fiddle with it and find the right software you can probably get sms alerts, connect it to your home security system, keep it in your car and make radiation maps of your town etc.

Its not fast enough to use like a metal detector for finding rocks, but you can use it to check out your house and suspicious objects.