The new pension bill (H.R. 4) requires a cancelled check or a written receipt from the donee organization to support any cash contribution.
Credit card receipts should also work. This takes effect for 2007.
These new requirements are in addition to the existing rule that gifts of $250 or more require a written receipt from the charity; for these gifts, a cancelled check alone isn't enough
Donation of non-cash property (clothing, furniture,etc.) in "fair" condition is effective as of the date (August 17, 2006) the legislation was signed by the President.