Many photographers use flash as the main source of light or to fill-in and add special effects. This post started with the look at Neil Turner's website, who does a lot of portrait work using flash. There you can see a series of techniques on how to use flash in such work.
If you are interested to know more about the flash and how it works, I suggest to take a look at a great article about Canon Flash. Nikonians can read more about almost the same subject in another articles: "Flash Photography with Nikon F4, Nikon F5 and/or other Nikon AF SLR models" and "Notes on using Nikon TTL flash".
You may be interested in more flash techniques, which are described by John Lehmann. He explains very interesting effects that you can do with flash. For those who just wants to start with basics I recommend the article "Greap Pictures with Flash" at About.com or "Flash Basics" by Guy Parson.
And here are several more links, which cover various flash usage:

• Controlling the Background Exposure
• Flash in Nature and Landscape Photography
• Smart Flash Outdoors
• Flash in Wedding Photography
• Bird Photography Tip #16
• Wildlife Photography and Fill-in Flash
• Another article on Wildlife photography with Flash
• What is the best flash for macro photography?
• Cross-Polarization Flash Macro Photography
• Novoflex Macro Flash Lighting System