I shoot with the Canon 5D Mark II, and it doesn't come with a pop-up flash built in. When I refer to my Speedlite, it's the seperate piece you see on the right. I use the Canon 580 EX II. (Sorry I didn't grab the link - I have a sick kiddo upstairs). You can find it if you Google B&H Photo.
Now I have it on my camera. See how it can swivel all around? You can bounce the flash off the ceiling, off a wall, a window, a glass door...anything. It makes the lighting much less harsh, and helps prevent that washed-out person/dark icky background look.
Now for some examples. For this first picture, the flash didn't fire. Yes, I could have changed my settings to let in more light, but in doing so I would have compromised picture quality. (I would have had to bump up my ISO). I could also probably fix this photo in Photoshop, but let's see what it looks like with flash. (This was taken with the Mark II).
This one was taken with my Canon Rebel, using the pop-up flash. YIKES!!! Look how flat Livia's face looks, and she has tiny pin-lights in her eyes. Gross! And she's not happy about being flashed right in the face.
Now back to the Mark II with the Speedlite. I bounced the flash at a 45 degree angle behind me. See how her face looks more like a sphere; more 3-dimensional? Compare to the picture above where her face looks flat and 2-dimensional. Now look at just her eyes; do you see how with the pop-up flash they almost look sunk back into her head? With the bounced flash, they appear more 3-dimensional as well. Pretty incredible, huh?
Here is the pop-up flash on the Rebel vs. the Speedlite on the Mark II, side by side to compare.
Really no comparison, right?
I'm sorry this is so quick; I'm exhausted! Someone feel free to jump in with a comment: there's also another very good Canon Speedlite...the 430 something? It's a bit less pricey, but I've heard it's a very good flash as well. Feel free to leave that info in a comment - thanks! :-)
Hope this information helps! :-)