Travel

Travel Insurance
When traveling abroad, many people choose to take out travel insurance since most people don't have private health insurance that covers out of country emergencies and healthcare. Travel insurance will also cover the cost of any problems caused by weather delays or cancelled flights. A lot of the airlines offer this insurance as an added charge. Make sure that you read the fine print to see what is covered and what is not covered (ie, Extreme sports or even winter sports are often a separate policy).  
InsureandGo
StaySure

Travel Safety
Government Travel Advisories and Alerts
SafeTravel -  know the penalties for speeding in various countries!

Airlines in Europe
List by Country

Finding the cheapest fares
Kayak
Opodo

Discount Airlines 
Ryanair
Easy Jet
Aerlingus

Rail
UK National Rail
Rail Europe
Eurostar (High speed London to Paris/Brussels/Amsterdam)

Bus
City-to-city / country-to-country transport at low cost
You can find multi-day travel passes at deep discounts
UK Megabus 
National Express

Car Hire
Europcar
National (same as Europcar, usually cheaper)

Hostels / Accommodation
HostelWorld
HostelBookers

Apartment/Holiday Lets - when traveling with a group, this option is often times just as affordable and offers the convenience of having private quarters, multiple rooms, extra living space, and even a kitchen!

Make sure you use a trusted source. If the listing is too good to be true, it probably is! Be weary of rentals that require you to pay the full price before you arrive. It's common to pay a deposit of 10% of your rental at the time of booking, but I wouldn't pay everything without seeing it first! Most will have you pay cash on arrival, but this should be worked out with the landlord before arriving.

HomeAway
Wimdu

Searching for the following terms in google will provide up-to-date results for this type of accommodation:

Self-catering
Holiday Lets
Short-term Apartment Rental
Bed and Breakfast