Are you working for/with a non-profit organisation? If you are, you should take a look into “Google Grants”.
The scheme is designed to provide free advertising to charities through the Google AdWords system and also includes non-commission based Google Checkout.
If you’re accepted into the program, your organisation is entitled to up to $330 “per day” of free advertising! There are strict rules over the content and target of the adverts, but with a little work, you could be driving a lot of relevant traffic to your organisation.
I’ve worked with www.brunswickstreetmission.org in Halifax to implement this and will be helping them to use the service to increase awareness of the great work they do.