Bing Travel: the search parameters are very intuitive and flexible, which makes flight planning very easy. The price predictor is a great feature, but don't ignore your gut instinct.
If you like to gamble on your travel arrangements and use www.priceline.com then biddingfortravel is your best friend. Improve the odds of getting the hotel you want at the prive you want by reading through the message boards of other priceline users.
I would not even think about purchasing a plane ticket without checking this site. You just need to cross reference the plane type that the airline say that you will be flying on with the website and then you can get profile of all the seats on the plane. That way you won't end up with a non-reclining by accident.
With Yelp you have to be careful as there can be sabotaging or self-promoting "reviews". In general, only rely on Yelp when there is a critical mass of reviews and always read a cross section of positive and negative reviews.
Tripadvisor can be very useful when planning a trip. However, reliace on tripadvisor should be positively correlated with the number of reviews of the hotel you are looking at. Always read a cross section of positive and negative reviews.