If you've lost your laptop charger or actually carry around multiple computers, you might look into getting a universal power adapter like Innergie's recently released mCube Slim universal power adapter.
Measuring smaller than the iPhone, the company says the Slim is "the world's slimmest power adapter on the market" and has a switch that allows you to go from 15-17 volts and 18-21 volts output, which covers most laptop computers. You can also simultaneously charge a portable device--such as a smartphone or iPod--via the integrated USB port (not pictured).
To see if your laptop is compatible with the Slim (sorry, no Apple notebooks), Innergie has a "tip check" list. The mCubeSlim retails for $99.99.