One-time clicks: Oh! Speakup - The Oxygen Network
For every person who subscribes to their email list, $1 will be donated to the New Orleans Children's Hospital.
Daily clicks:
The Ecology Fund

Please let me know about any other sites you've found and I'll add them to the list. I hope to add on some info about national and international charities and volunteer opportunities at a later date, but it will take a bit of time to compile a list.
Thanks, all!