"Tax-free money" - scary??

There is no such thing as "tax free money"! If you receive an expense allowence for travel, meals, housing, etc, you must demonstrate to the IRS that these expenses were unavoidable due to working away from your established home (basically duplicating your home expenses), and then you must present documentation of those expenses in the form of receipts.
Anything for which you do not have a valid receipt will be taxed.

So, if you are paid $10/hr. for your work, and "given" $1200 for expenses, you must use the $1200 on travel and the cost of living away from home, and document that, or it will be taxed as well.

It is not scary if it is used correctly. To be eligible for this money you have to have a permanent tax home. If you have a tax home then it is good and legal. Don't listen to the companies that tell you "if you have a permanent mailing address you are eligible" because you are not. If you don't have a tax home then you need to let your company know and make sure they tax you appropriately. (otherwise it will come out when you file at the end of the year or, if it doesn't, you will be worried about that potential audit and charge at that time)

There are some fantastic articles on this subject. Check out the current issue (feature article is "Tax Talk for Travelers) of Healthcare Traveler at www.healthcaretraveler.com or use the links to the left on their home page to read about taxes and traveling.