If you want to capture really good sports photos, and you're using a digital SLR that doesn't have a built-in vertical grip like the professional level Nikon D3, then you you need to get one for the ergonomics of shooting good sports photos. A vertical grip allows you to easily shoot in portrait mode because of the shutter release that's installed on the grip. Since most sports subjects are human (tall from top to bottom in the view finder), the portrait orientation of your photos will capture the action better than the landscape mode.
If you're shooting with a Nikon D300 or D700 get the Nikon MB-D10 vertical grip. If you have a Nikon D200, use the MB-D200 vertical grip. If you're using a Nikon D80 or D90, get the MB-D80 vertical grip. If you're a Canon shooter, and you use either the 30D, 40D or 50D, get the Canon BG-E2N vertical grip. If your Canon body is a Digital Rebel XT or XTI, get the BG-E3 vertical grip.
The other big benefit of using a vertical grip is the additional battery it allows you to use to give you more shooting time without recharging.
