Yellow Tags combines stickering and geo-tagging. Users place a yellow-arrow sticker somewhere with a unique number on it and write an entry about the place they left the stick. Then people can SMS yellowtag.org with the unique number on the sticker to receive the message. So far it’s only available in NYC, but they have plans to expand.
I guess this is old news in the blogosphere, but I just found out about it, via Trendcentral.