here are a couple resources for you.
Check with the IRS for a searchable online listing of organizations eligible to receive tax-deductible charitable contributions.
For more consumer guides to charities check out the Federal Trade Commission's resources.
If you haven't decided on a charity yet, the Better Business Bureau has a search as well. From their site, you can see how the charity spends the monies they receive - for instance the percentage of their funds spent on Programs, Fund Raising, and Administrative costs.