Air-Car vacation packages
Buying your airfare along with a rental car at the same time in the form of a fly-drive vacation package often saves you money over purchasing each separately
If you do plan to rent a car, it's usually far cheaper to buy one of these vacation packages than it would be to get plane tickets, then go about arranging a vehicle separately.
This is largely because the outfits that package these vacations are consolidators of at least the airfare portion (sometimes the cars as well), meaning they can sell the tickets at a discount because they buy them in bulk from the airlines way below market value.
The practical upshot is not only can you get to Europe pretty cheaply, but once you're there you have your own set of wheels so you can properly explore the continent's nooks and crannies.
The best places to get air-car packages
Auto Europe (www.autoeurope.com) - Since they moved from doing just cars to covering cars and airfare (and more), AE has become one of the most consistent and reliable sources for both inexpensive airfares and barrel-bottom car rentals. It goes without saying that they can pair the two cheaply as well.
Go-Today.com (www.go-today.com) - The price champ on air-hotel packages also offers cheap "Self-Drive Vacations" deals in most countries of Western Europe.
Gate1 Travel (www.gate1travel.com) - One of the consistently cheapest tour providers around. They cover the entire world—though most air-car packages are in Europe, North America, and a few key places like Costa Rica.
Untours (www.untours.com) - Sort of like the Platinum Edition of a vacation package (at least for Europe), taking care of all the major costs, details, and logistics but leaving you to plan your daily sightseeing and travels. For one price, you get airfare, some form of transport (rental car or train or bus pass), lodgings (usually an apartment or rental villa), and a local contact who will help you get settled in, show you the ropes, then be on-call to help with questions. You also usually have one activity during the course of the week when all other "untour" participants in your area are invited to get together.
Tico Travel (www.ticotravel.com) - Go-to packager and tour company for Central America and South America, based in Costa Rica.