Air Travel: Kayak.com

Kayak.com is a great site that searches travel sites and airline's own booking sites for the best deal on airfare for you. This can be very convient for planning a trip, because instead of search all the sites yourself, you can search all of them from one place. Another cool feature of Kayak is it's "buzz" section where you enter your starting point and a region as a destination and search for the cheapest fare. This is useful if you want to go to, say, the Carribean but aren't picky between Barbados and Trinidad.

On caveat on air travel: the web is rarely the cheapest place to buy plane tickets, travel agents are! It's hard to believe, but travel agencies such as Fujita World that specialize in one sort of travel can often get really good deals on airfare. It's not obvious to me how they are able to do this, but roundtrip from San Francisco to Narita at $480 in August, is the best deal you can find, hundreds of dollars cheaper than united's website. So if you are planning a long overseas trip, I'd advice contacting an old fashioned agent who specializes in these sort of trips.