The first geocache was hidden on May 3rd, 2000 in Oregon, United States by Dave Ulmer. He placed a 5-gallon bucket that contained various items including Delorme Topo USA software, videos, books, a can of beans, money, and a slingshot. The original geocache is no longer there, but a plaque has been placed to honor it. This marked the beginning of what we now know as geocaching.
Additional interesting geocaching history listed here. Check it out. How geocaching.com started.