Look before you leap.
Kentucky and several neighboring states have many downed trees at present, due to Hurricane Ike which passed through last fall and the terrible ice storm we suffered last winter. I've noticed ads in the newspaper that say, "Free firewood, you cut and haul." People want some free help with cleaning up fallen trees.
Since firewoods are not equal in fuel value, it would be a good idea to identify the species of the fallen tree before promising to cut it up and haul it away. (And don't be surprised if the property owner doesn't know or is mistaken about the type of tree.)
Remember that firewood should always be burned in the area where it's cut to prevent the spread of diseases and pests. In particular, there are laws and quarantines for ash firewood so be sure you stay within the law.