SB-400:

• Outdated.
• Entry level speedlight. Compact, simple and affordable.
• Designed to be on the camera and inability to bounce light sideways.
• i-TTL (intelligent through-the-lens) only with cameras compatible with Nikon CLS (creative lighting system).
• Not compatible on film cameras.
• Not compatible with Advanced wireless lighting.

SB-600:

• Quite a decent capable compact unit with plenty of features.
• Can be used as a slave when activated by the camera pop-up flash as a commander.
• Compatible with i-TTL, D-TTL and TTL (film based).
• Decent flash coverage, including Wide area AF Assist Illuminator.
• i-TTL (intelligent through-the-lens) only with cameras compatible with Nikon CLS.
• Advanced wireless lighting only when used as a remote unit.

SB-800:  I wouldn’t discuss this model, simply since these are no more in production and discontinued from Nikon. Nevertheless you may find cheaper in ebay and if you want to buy it’s quite a upgrade from the SB-600 and a great power horse.

SB-900:

• High end top of the line unit with advanced speedlight range, a replacement to the SB-800.
• Large illuminated LCD panel.
• Distance and Aperture priority mode helps shooting wedding and product shots by giving maximum power output.
• Compatible and auto-detects the Nikon FX and DX format and select suitable light distribution angles and pattern (standard, centre & even)
• AF-assist illuminator for multi-point AF with wide 20-105mm focal range.
• Auto Zoom range of 17-200mm.
• Multiple flash exposure modes (i-TTL, AA, A, M).
• Repeating flash for creating stroboscopic multi-exposure effect.
• Compatible with i-TTL optimized for use with Nikon CLS and Advanced wireless lighting upto 3 remote flash group.
• Dedicated color gel filters to correct white balance.
• Heavy and expensive (price to drop with the release of SB-700 and SB-910).
• Allows firmware updates with select Nikon DSLR.

SB-700:

• Recently introduced unit with plenty of features and designed for ease and intutive operation.
• Major improvement over the SB-600 and almost challenging the SB-900 (tackling the issue of automatic shut-off on over-heating).
• Compatible and auto-detects the Nikon FX and DX format and select suitable light distribution angles and pattern (standard, centre & even).
• Automatic temperature regulation via slow recycling time.
• AF-assist illuminator for multi-point AF with wide 24-135mm focal range.
• Compatible with i-TTL optimized for use with Nikon CLS and Advanced wireless lighting upto 2 remote flash group.
• Assortment of accessories (more than the SB-900) right out of the box.
• Dedicated hard color filters, these are actually better than color gel filters which fades with time.
• Automatically identifies hard type color filters (tungsten and fluorescent) and corrects white balance.
• Light and affordable.

SB-910:

• High end unit with a slight improvement over the SB-900 on tackling the issue of automatic shut-off on over-heating and out of box accessories.
• Slight technical improvement over the SB-700.
• Major improvement over the SB-600 and almost challenging the SB-900 (tackling the issue of automatic shut-off on over-heating).
• Compatible and auto-detects the Nikon FX and DX format and select suitable light distribution angles and pattern (standard, centre & even).
• Automatic temperature regulation via slow recycling time.
• AF-assist illuminator for multi-point AF with wide 24-135mm focal range.
• Auto Zoom range of 17-200mm.
• Multiple flash exposure modes (i-TTL, AA, A, M).
• Compatible with i-TTL optimized for use with Nikon CLS and Advanced wireless lighting upto 3 remote flash group.
• Assortment of accessories (more than the SB-900) right out of the box.
• Dedicated color gel filters to correct white balance.
• Lighter than SB-900.
• Allows firmware updates with select Nikon DSLR.
• Expensive then the SB-900 and SB-700, price in Amazon is $585.
• Available on pre-order (expected to ship in the 3rd week of December 2011).