Google offers free credit card processing

Google checkout is a "pay-pal like" service that allows websites to accept payments with minimal effort. Nonprofits can use it to collect donations from supporters without having to set up a credit card merchant account.

"Google Checkout for Non-Profits is a fast, convenient donation process that helps you attract more donors and increase online giving to your organization – and you can use it to process donations by charging credit or debit cards for free until 2009"

I've looked through the program and it appears that you do need to be designated as a nonprofit with the IRS (501c3 - or similar) - so some smaller groups may not qualify - but it seems like a good option.