Give it away

It's good to support worthy causes.  It's also good word of mouth.

The key word in "non-profit organization" is "organization" -- these are natural communities of highly connected people who talk to each other.  If you provide genuine support to a good group, all of their members will know about it. 

Pick a place where you can have an impact. No one will know about your donation to the Red Cross, everyone will know about what you do to help a local student group or community center.

A great example:  Email provider Vertical Response lets non-profits send their newsletters for free.  It's a huge value to the organization, and every member knows exactly how the newsletter was sent.