Here are some of my favorite general travel planning resources, by category (and I will update as I get new favorites):

General travel:
TripAdvisor – take with a grain of salt, especially in the “forum” area, but it’s a great way to get a general sense of how a hotel is regarded, get room tips within those reviews. In the forum section, you can often get specific answers that you haven’t been able to find elsewhere. However, be advised that the regular posters in such forums can get very testy if your questions can be answered by using the “search” function in the forum. Also, I’ve noticed that the cranky/helpful spectrum varies widely. I advise using the search function thoroughly and reading recent posts for a while before posting a question of your own.
Rough Guide – a visually stunning travel website that is especially useful for destination-specific prioritizing. Easy to click through their “must-see” photo albums for each country, and some destinations have suggested itineraries based on interest. It’s a great starting point.
Frommer’s – great guidebooks; website is useful for suggested itineraries on a limited timeframe or budget. Helpful hints on how to save money & time. Most destinations have a suggested family itinerary that can be a good starting point.

Logistics/flights:
U.S. State Department – always worth checking travel warnings, visa requirements etc. before proceeding. Many countries require obtaining sometimes onerous and expensive visas weeks/months in advance (e.g. China, Brazil).
Google Maps – I use this ALL. THE. TIME. to check distances, see street view of lodging possibilities, see topography (i.e. walkability), travel options (it gives options for driving, public transportation and walking), etc.
Rome2Rio – a fun way to compare transportation options; verify cost, but it’s a great starting point. Be sure to put in your dates and the number of people (makes a big difference comparing driving to buying plane/train tickets!).
SkyScanner – flight/hotel comparison site that includes budget airlines.
Hipmunk – flight/hotel comparison site that is visually easy to look at and understand. Sorts by “agony” (time/cost calculation), which can be very helpful.
The Man in Seat Sixty-One – Great guide to train travel; thorough, helpful. The guy who writes it actually posts on TripAdvisor as well.

Lodging:
AirBnB – proceed with caution but can yield much better value lodging than traditional hotels or B&Bs, especially if more than two people are traveling (many hotel rooms cannot accommodate a family). Advice: choose options that have MANY very positive reviews, avoid those who have more than one or two reviews that state “The reservation was canceled X days before arrival. This is an automated posting” – this means the host canceled the reservation, leaving the renter scrambling to find alternate lodging. I also recommend sending a personal message to the host, verifying their pricing given your dates, especially if it is far in advance and/or during a high demand period (e.g. New Years). May also be worth asking if there is a price break for kids regarding the “extra guests” fees. AirBnB can be an especially good value in cities, where hotels tend to be very expensive for tiny rooms.
ReidsGuides – excellent content with fun, unique lodging options and good advice from a very personal perspective.
CastlesandPalaceHotels – great website for Castle hotels in many European countries, organized by country and budget with nice descriptions from a personal perspective.
HomeAway, VRBO, FlipKey – these are not as slick as AirBnB (the newer kid on the block), but depending on the destination, these may have more offerings and are generally more established.
SabbaticalHomes – for longer stays in a furnished home but worth looking at regardless; some allow short stays.