I'm not big on causes, but I did hear about this one, which I think is worthwhile. They don't ask for any money; just a moment of your time. The website, vietnamdogtags.com, was started by Stacy Hansen, who purchased a number of American military dog tags while in Vietnam. She's looking to return them to the owners or their families. She's already managed to return 300. If you served there, lost a family member there, or know someone who did, just take a minute and check the list of names.
If you have dog tags, you can also submit them to her to be included on the list.