This one (Ergotron LX Desk Mount LCD Monitor Arm, Tall Pole) has been working well for me for the last couple of years, in conjunction with a Conset sit/stand desk (also great). I got the Tall Pole version just to be on the safe side, and it gives a lot of flexibility on where you want to position and angle the monitor.
Just searching the Internet shows that there are dozens of manufacturers for these things and price ranges vary widely. You should have no problem finding one.
If you're a tall guy like I am, I'd go for the highest one I could find. I have it almost at the highest setting when sitting down and I wish it was higher when standing up.