The limit for deductions on your federal income tax depend on the type of property you are donating and to whom you are donating it. In general, you can't deduct more than 50% of your Adjusted Gross Income. However, you may be limited to only 30% if the organization is not a "50% limit organization," or to only 20% if the property is a certain type of appreciated security, but there are exceptions to these limits. Take a look at Pub 526 - it explains all the gory details:
Publication 526 (2007), Charitable Contributions
I can't answer your question regarding state taxes, since you don't tell us what state you live in.