Twenty years from now you will be more disappointed by the things that you didn't do than by the ones you did do. So throw off the bowlines. Sail away from the safe harbor. Catch the trade winds in your sails. Explore. Dream. Discover. -- Mark Twain
Update Aug 22nd, 2012:
GetGoing.com is a YC startup that allows leisure travellers who are flexible in terms of where to go to get up to 40% in discounts on airfare.
Often, I just want to travel. Just get away. Go some where new. If the price is good and I’m free, I’ll go. I don’t even really care where. For people like me, here are some websites that allow you to do that.
Adioso is a website for adventurous travelers who want to take vacations and are flexible on days or even places.
Adioso allows you to search flights with complete flexibility, like if you want to go somewhere in Europe in November but are not committed to specific area or time. Examples of the natural language-enabled broad or open-ended searches specific to Adioso:
Chicago to Boston next week
San Francisco to Europe late September under $800
German Wings Blind Bookings allows you to buy a flight and find out where you’re going on that day.
Surprise flights at guaranteed fares!Simply select your departure airport and the type of holiday you want by clicking on “Party”, “Snow and Skiing”, “Shopping”, “Christmas Markets”, “Metropolis Eastern Europe”, “Metropolis Western Europe” and “Culture”.
Kayak has an experimental site called Explore
Last Minute has its own secret flights scheme
Unfortunately, it's more similar to "top secret hotels" in that you know where it is, just not which hotel. In this case, you know which city you're going to but not which airline or airport.
German Wings seems to be the only operator that truly has secret flights that you book but don't know where you'll be going. If you've heard of others or tried one of these, please comment!