I picked up running that last ~year or so after my injury and really love it - it's amazing how in-shape you think you can be from lifting weights only to find out how quickly you get winded running :) at least for me....
Anyways, I think Polar is pretty much the main brand - I currently have
THIS one which is a pretty basic heart rate monitor, it can do a few extra features but to be honest I don't use them. I'd like to get
THIS one, which has GPS on it as well.
Another nice thing about it is it works with a lot of the major brand treadmills so you don't even have to use your wristwatch.