A number of websites can help guide you through the planning process, allowing you to collaborate with your traveling companions — your spouse, older kids, siblings, or friends.
Start a vacation plan on Triporama, which starts you with a list of tasks that you can assign to the other users you've invited. You and your co-planners can message each other, take polls to make decisions, create a printable itinerary of daily events, and get reminders. You can bookmark your travel research, and the itinerary includes a very helpful map so you can see at a glance your realistic your plans are. (Really, you're criss-crossing town five times on Monday?)
Trooptrip does pretty much the same thing as Triporama, just in slightly different ways.
Tripobox is worth checking out if you happen to be traveling to Berlin, Miami, Paris, Amsterdam, London, San Francisco, Chicago, Barcelona, or New York; the online vacation planner includes deals in these cities, with plans to add more cities as they grow.
App planners offer fewer bells and whistles, but with a more streamlined approach.
KAYAK (iOS, Android, Blackberry, Windows Phone; free) The app for airline price aggregator KAYAK.com searches for flights, car rentals, and hotels, helps you manage your itinerary, and sends you flight status updates.
TripAdvisor (iOS, Android; free) is the companion app to the online travel search website and includes reviews.
Need help organizing what to bring? You can compile lists with the above planning tools, but a few apps get you started packing with a list of items — in case there's something you haven't thought of.
Packing Pro (iOS; $2.99) Build customizable packing lists from a menu or items you add, then edit, sort and share.
Stow ( iOS; $1.99) Create packing lists from pre-populated templates, which can do things like take destination weather into account, then share them via email.
Packing List (Android; free) Create packing lists from scratch or from previous lists, then share them.
uPackingList (iOS, Android; free) Create packing lists from a catalog of items, then share them via email.
Now that you know where you're going, how, and when, there are a few last details to sort out. For example, what can you bring through security?
Check out My TSA (iOS, Android; free), which not only tells you what you can bring, but also gives you updates on how long the lines are.
Many airlines have apps that let you track flights and give you updates on delays, but there's also GateGuru (iOS, Android, Windows Phone; free), which gives you real-time updates on security wait times, flight delays, gate changes, and layover times.
If you're traveling by car, you'll find yourself with a different set of problems, some of which can be answered with apps.