Portrait Lighting Equipment
If there is one thing that makes an image, it's the lighting. Be it natural, reflected, or artificially created with a strobe (flash), this is the most important aspect to portrait photography. Even with the most expensive cameras and lenses, a photo is nothing without light. You must learn to control and use the light to your benefit. These few items here are only meant as a guide to get you started.
2-3 Flashes (Speedlights) with manual control - For portraits, it is a good idea to have at least one flash. Indoors or out it will allow you to bounce off of walls or ceilings or provide fill flash on a sunny day outdoors. By using a couple flashes off-camera, you can control the intensity, direction, and shape of the light on your subject. With enough flash power, you can even overcome the sun and light your subject strictly with artificial lights for very dramatic portraits. With a flash with manual controls, you can adjust the power of each flash individually to control the intensity. This will be important when we move the flash off the camera. The flash will lose the ability to communicate with the camera and meter automatically. It is now your job to think where, how much, and how the light will fall by manually controlling it.
Reflector - A reflector is a cheap and effective way to reflect back natural or artificial light back towards your subject from a different angle as where the light originated. Here's an easy tutorial on how to make your own reflector.
Flash Modifiers - This one is up to you. It can be as basic as an umbrella, DIY snoot (light tunnel), grid spot, gobo (go between), or bounce card or more advanced such as a beauty dish, ring flash, or softbox. This is where your artistic creativity steps in and you decide how to light your subject.
Wireless Trigger and Receivers - Professional portraits are done with multiple lights, off-camera. Now that you have several flashes set up, you need a way to fire them all at once. This is where a wireless transmitter/trigger and receivers (one for each flash) come in play. The transmitter sends a signal to the flashes to fire simultaneously when a photo is taken. Professional radio triggers (PocketWizards) can run up to $200 a pop. They are expensive because they just work 100% of the time. They are the gold standard when it comes to triggering remote flashes. Cheaper alternatives can be had for about $40 on eBay or from other companies (AlienBee's CyberSync Triggers and RadioPoppers). They may not be as reliable, but for the money you save, you can learn to live with them until you learn more and become committed (addicted) to off-camera flash.