Super cheap, super easy. Use a decent wattage resistor, 1/2 watt or more, or calc out the dissipation. The circuit isn't particularly efficient (3v drop) but it is cheap and easy!
Basically the LM317 acts as a variable resistance limiting the current. The chip is rated for 1.5A, I'll say that if you are driving more than 500mA you NEED a heat sink. These are great with a 1.2 ohm sense resistor and connected to 12-14v, it's perfect for driving the 10 watt white LEDs (1A@9-12v) you can get for a buck or two on ebay. The LM317 is one of the cheapest components you can get, and one of the most useful too. Buy a pack of them, you are going to want more.
This feeds into the LED, and you switch the ground connection with an NPN transistor to provide PWM.
I've made a bunch of these.. if quick and dirty is good enough, it'll do the job nicely 
Here's the Application Note from ON Semiconductor on using the LM317 this way: