To be a valid contract (in US law), the contract must contain both an offer and consideration. This means that something must be exchanged. If I drew up a contract saying that I would pay you $1000 tomorrow and signed it, I would have no legal obligation to actually do it because it is not a legally binding contract. There is no real legal distinction between working for goods and money; consideration is consideration no matter what it is.

The five things that must be in a contract to be valid are:
Offer
Acceptance
Consideration (the mutual promise given the the offered)
Mental competency
Legal subject matter

So if you sign a contract to do work for them with nothing in return, it is not enforcible, either party could walk away at any time.