I'm left handed as well, and the best grip I've found so far is the Ergo Grip "Sure Grip" in Ambi format. Notice how the grip extends up the back of the lower, effectively moving your grip rearward and filling your hand more.
[img]http://www.ergogrips.net/smEgAR.jpg[/img]
It's also available in left handed, if you don't share with any righties.
It's what they use on the SPR. [:)]
Here's the manufacturer's [url=http://www.ergogrips.net/ar15.html]website[/url] but I've seen them a bit cheaper, and on a more user friendly website, I just don't remember where.
There's also the Fobus G27, but I don't know as yet if it's Ambi or Righty only. I was coming over here to ask that very question when I saw your post. It's plastic, and also extends up the lower, but it has more pronounced finger grooves and serrations.