Are you looking to get a little more specific with your flight search?  When is the cheapest time to travel?  When is it the right time to book your flight?  Or, maybe taking the train to your destination is the best way to go.  (Kayak.com/explore) is definitely a fan favorite.  It lets you enter your departure city, season in which you want to travel and maximum flight time.  It then quickly calculates all your options and displays a map showing where you can go within those parameters.

Want to see if the train is less expensive then try (Hipmunk.com).  This great site includes Amtrak prices and departure times when you search for flights on a specific route that is also served by the railways.  This  site could lead to great alternatives for trips such as New York to Boston or Chicago to Milwaukee.

Google also recently came out with there own version for searching flights (google.com/flights).  While still new and lacking a few features, it still makes for a great tool to check round-trip tickets for most domestic destinations.

One last website which is perfect for checking to see when the right time to book is by Microsoft (travel.bing.com).  They have a great feature which helps recommends to bookers on whether they should BOOK NOW or WAIT.  Although these recommendations are not always 100% accurate, they can definitely help in deciding when to book your next business trip or vacation.