The Triborough Bridge
The Triborough Bridge connects Queens, Manhattan, and the Bronx. It runs from the northwestern tip of Queens to 125th St. in Manhattan, with an off ramp leading into the Bronx. From Queens, you can reach it via the Grand Central Parkway, which will get you from LaGuardia Airport to Manhattan in about 30 minutes (without traffic). You can reach it in Manhattan via the FDR Drive and in the Bronx via the Major Deegan Expressway.
It costs $3.50 each way by car.