Which external flash unit is suggested for amateur DSLR user?
I have a Nikon D5100 and often use it with my 35mm f/1.8 lens, but when I use my kit lens for certain situations, the built-in flash is too harsh and bright. Which external flash would you suggest?
If flash is too bright, step back. If too dark, move forward. If you want to get an external flash, check your camera's manual for compatible models. I guess it's pretty safe to say all modern flashes from Nikon should work. Get one that fits your level of competency. If you shoot at full Auto most of the time, get a flash with TTL features. If you prefer manual mode then any cheap generic manual flash will do for you.
I have found usually the middle of the road flash unit work's best for home use. By that I mean the manufacturers usually offer 3 models, a low, medium, & high power unit. I found that he smallest/cheapest flash is just not powerful enough, while the most powerful is usually too big to "lug" around. You can also find flash units made by companies other the one by your camera manufacturer.
I like to use a Lumiquest pocket bouncer with my flash as when you point your flash to the ceiling with the Lumiquest attached the light bounce and ends up more flattering for indoor shots like your would find in the home. Also if you take pictures of people in bright sun with the sun to the side or to the rear of the subject the pocket reflector will bounce just the right amount of light into their faces.
Note when looking at "cheaper" flash units make sure they are designed for digital cameras! The reason is that older flash units used with film cameras have ore voltage and will burn out the circuits of a digital camera.
The short answer is the SB700. The little SB400 is not bad, but as you work with bounce flash you will probably be coveting more power and adjustability. The SB900 is supposed to be great, but for most amateurs what it adds beyond the SB700 may not be useful enough often enough to be worth the added expense.