Note: Marco Polo has been sold to Libera Inc. and I will no longer be developing the Marco Polo app. I'm very excited to see what they will do with it! (2 May 2013.)
Original post (29 Oct 2011) follows:
The app uses some neat features like HTML5 geo-location and localStorage, to remember your last location. Plus, the app is mobile compatible! So it will even work on iPhone or (hopefully) BB6 - any webkit browser.