@HiddenCash: The nation's newest obsession

It all started in San Francisco: an anonymous millionaire began hiding $100 to $1000 in plain white envelopes across the city and posting clues of their whereabouts on Twitter. Not only did he start a social media frenzy, but copycats are popping up all over the world, from Durham and London to India and Nigeria. Says the (also anonymous) UK benefactor, "it's fun for me and it also helps people, so it's a win-win." Here are a few more examples of mystery donations that have warmed hearts in recent years.

Tips for Jesus

Anonymous restaurant and bar patrons leave absurdly large tips for the waitstaff, sometimes up to $10,000. The premise of the act is, what would Jesus tip? The group touts generosity and tweets receipts on their account, @tipsforjesus.

Salvation Army Secret Santa

Every year, a donor gives $50,000 to the Salvation Army. Watch the video and read more here on ABC News.

Dartmouth College

In April of 2014, the Hanover, NH university received $100 million from a donor, the largest amount from a single giver in their 244-year history. Read more about this generous (and completely anonymous) donation here on Bloomberg.

Kentucky Cancer Patient

After Addie Roberts' family was scammed out of money meant for her cancer treatment, an anonymous donor stepped in to help cover the costs.

The message behind most of these donations is to pay it forward: when you have something to give, give it freely, even if you don't get any recognition. The knowledge that you are brightening someone's day (or year, or entire life) is worth everything.