travelfree.info - on this site you can find offers for buying tickets to various directions, promotions and discounts for air and bus companies, car rental, and accommodation, which are collected and constantly updated. The particular dates and how booking instructions are indicated. Even if you had not thought about a trip before, it is easy to quickly catch on to the idea.
skyscanner.com - aggregator for airline ticket searching. The option to fly "Everywhere" is a good source of inspiration for choosing a direction.
greatescape.co - an indispensable assistant when the vacation dates are known and the budget for tickets is clearly defined, but it is difficult to choose a destination.

For airline tickets in this category, the already mentioned travelfree.info and skyscanner.com win confidently. In the latter, the price calendar also works great, you can compare where it is better to book: sometimes on the site - the aggregator, and sometimes on the official website of the carrier company.
To search for a bus-train combination, we use the Rome2rio service - it indicates all possible combinations to get from point A to point B. Further, for booking, we go to the official websites of the carrier companies.
For bus rides we use Omio. Most frequently we use the following coach companies LUXexpress, Ecolines, Flixbus. 
For ferries in our Baltic states region - Tallink.
Sometimes you can use the Blablacar service and book a trip with a private car. It's almost always cheaper, but do read the driver and other passenger reviews. They also recently added BlaBlaBus - great value for money!
Another great site is Holidaypirates, on which you can often find rare deals for 2in1 holidays - flights plus hotel.

Most often, we use booking.com and airbnb.com to search for accomodation. If you have not registered on these sites, click on the links below to receive a discount on your first reservation! 
10% for your first booking.com reservation: https://www.booking.com/s/34_6/m0rti004 
Up to 42 EUR discount for your first Airbnb reservation: https://abnb.me/e/8RUJyFM7YZ
When searching for an overnight stay on a camper trip, we open Park4night and watch reviews on a particular location.

To the questions "What, where, and when to look near me?" google search is best. Less commonly Google maps, maps.me, theculturetrip.com, tripadvisor.  
For tips on events from local people or just to meet and get acquainted with the culture of the country from the inside, we go to couchsurfing.сom.
Pocketguide is good if you like to perceive information by ear - these are audio guides with maps. 
On inspirock.com you can set the duration, direction, theme of the trip, and look at the proposed plan, which, if desired, can be easily edited and printed.

We download offline maps.me maps - it’s convenient that there are categories and you can find a lot of interesting things right on the map. You can also bookmark it and write a comment on the desired point.
If you need to get to a certain point on foot or by car, you can get directions both in maps.me and in Google Maps. But on maps.me check the route for unexpected ferries instead of bridges, its navigation capabilities are somewhat limited.
Another good offline navigation program is Sygic. Its advantage is that it allows you to choose the type of road that you want to go on: paid, free or gravel.
We use Google Maps and moovit if we want to travel by public transport. Bolt, Uber, Gettaxi - if you want to take a taxi. Using promo code N9D34 during the first trip using the Bolt service, you also get a discount of 5 Euros. You can register in the application following the link: https://invite.bolt.eu/N9D34 In some countries, you can also rent an electric scooter through this service.

Before the trip, we always check the weather forecast for the entire duration of the trip on weather.com. Sometimes we look at statistics for the past years of weather in the place where we are going, opening articles about specific cities on Wikipedia. According to the forecast, we pack appropriate shoes and clothes.
We check the average price of products before traveling on globalprice.info, so that it is more convenient to plan the budget and, perhaps, to grab something from home.
In order to clarify whether any special vaccination is needed, we check 
wwwnc.cdc.gov/travel/destinations/list
Do not forget to pack a first-aid kit into your luggage! Our universal list of things to take on a trip will soon be available for download from the site here.
Travel insurance for us is already included with our credit card. It covers the risk of flight cancellation, insurance of a rented car and much more. We recommend finding a similar offer in your area.
The wifi map application helps finding where the Wi-Fi was theoretically, but to connect and find the password you still need to spend some mobile data in roaming.