For 2006, you need a receipt for individual donations above $250. For less than that, the IRS will need to see some proof – a cancelled check will usually suffice for the small-dollar stuff.

The $500 limit is the point at which you must give more details of non-cash contributions on a separate form. That is a global limit, by the way.

The others have wisely observed that you must be able to justify your claim for a deduction. Please be aware that the IRS will be auditing charitable contributions closely this year and next. If you have not kept receipts and do not have easy access to cancelled checks, it would be best not to take the deduction. Unless you write out dozens of small checks for donations each week, it is not going to make a significant difference to your tax position.

New rules. You must have receipts for all charitable contributions. You don’t have to attach them to your return, but they must be kept available in case of audit.

You can claim less than $500 without having to send proof or fill out any additional forms, most of the time, but if you are audited there are specific forms of proof that you must show.
IRS Publication 526 explains all of this.

No, the rules always said you could only claim what you gave, but now new rules say it’s zero without receipts.

0 Dollars.