Neat concept sites.

I posted something about geocaching a while ago. It's a pretty neat idea, but there are also other websites with similar ideas and they don't require the overhead of buying a GPS unit.

• BookCrossing.com: This site is an online book swapping site. Basically people leave their used books somewhere and post the details of it online. People go to the site, and the first one to the location will get the book. Then once that person has read the book, they in turn leave the book somewhere and post the details of it online. It's a neat way to get a free used book.
  
• Where's George?: Track the history of American money.
  
• Where's Willy?: The Canadian version of Where's George? I found a $20 with the URL of this site stamped on it. I checked out the website and discovered this neat concept site. You can start tracking your bills simply by writing the URL of this site on the dollar bill of your choice, and remember the serial number of the bill.
• Infiltration.org: This is a website dedicated to helping people get to places they shouldn't be in. It may not be entirely legal to do these things, but it sure it fun to read about other people who have gone on infiltration adventures to abandoned subway stations, etc... Check it out!