I've been a big fan of off-camera flash since I started using a Vivitar 283 and 15-ft sync cord in the mid-80s. Since then I've added many more strobes and some radio triggers/receivers to my lighting kits. The last few years, though, I've done most of my lighting with wireless flash built into my cameras and strobes.
Nikon started the trend with their "Creative Lighting System" and Commander mode capability in the D70 and the SB-600 and SB-800 Speedlights. Since then they've grown that functionality and added the SB-900 strobe and the SU-800 Commander module. What that means for photographers is the ability to do fairly complex lighting setups with a minimum amount of gear. I primarily use the Nikon gear, but Sony also offers similar capability in some of their cameras/strobes and Canon has a wireless mode too, though not supported through the on-camera flash (though that sounds to change with the introduction of the 7D).
In August I taught a photography workshop (Colorado with American Photo) with Dave Black, who's a master at this stuff. Dave showed a couple of neat tricks to enhance what he was doing with Nikon's gear. The first was utter simplicity, and involved getting more power to the subject. The easiest way to do that is adding a second strobe, and I've done that before with a bracket that holds two strobes. Dave made that even easier, by simply rubber-banding two Nikon SB-900's together. No extra weight, and simple to separate for two light sources. The second trick was to use one of Nikon's TTL cords (now called the SC-29) to get the SU-800 Commander unit away from the camera and pointed to the strobes.
I was thinking of all this yesterday while doing some photos for a local air ambulance service. Working inside a helicopter on a sunny morning, I needed to create light inside the chopper to either supplement the existing light or overpower it. In each case I used a pair of Nikon SB-900's and the SU-800 to trigger them. I put strobes on the floor, hung them from the ceiling and even had an assistant hold one outside the helicopter, firing in to simulate sunshine. Many cameras/strobes today also let you use a shutter speed higher than the regulation sync speed (around 1/200 second). Taking advantage of Nikon's FP sync mode, I was able to photograph the pilot at 1/2500 second at f/7.1, darkening the blue sky to make the photo more dramatic.
I used to put a flash in my camera bag only when I knew I was going to need it. Now I keep at least one flash with me at all times. Being able to add or create light in almost any situation lets me make photos that most other photographers can't, without dragging a case of lights everywhere. I've come a long way since that first Vivitar strobe and sync cord, and never want to go back!