You'll get a lot of answers on this one. You may want to include your purpose of the loan: Primary residence or investment property. If investment property, are you planning on renting it or flipping it?
Remember that on a national average, home values are going down this year about 11%. If someone owes more than the value but you really want the house and don't care about the value, then ask the owner if their loan is assumable. You may be able to pay him/her a small fee to walk away from the house and you take over the payments. This is done in a title office with permission from the lender.
If you do care about the value, go to the courthouse for that county and check out the properties going into auction. You can find properties at 30 cents on the dollar.
Another option is to ask the homeowners how much they want for the house. Distribute a flyer in the neighborhood, "What will it take you to move?" If you get a call and the person is upside down ask him/her if the bank is willing to take a lesser amount and have the seller hold a second mortgage promissary note for the difference. There is no lien on the house. This allows the seller to walk away from a potential default and pay the difference of the shortgage.
Most people won't do the above, they just walk away.
Check out the site: netronline.com for tax assessment values and appraised values based by county. In most cases, you just need the address of the property.