WhatsUp Gold. Very good program, I used it for a while some time ago. Draws maps and what not as well, so you get a graphical view of your whole network and can isolate which link is down. Also lets you connect directly to a machine by clicking on it.
After several months of review, I'm recommending the purchase of SolarWinds Engineers Edition for myself at work. I have yet to find as comprehensive a package as this product offers. I'm also opting for the Orion Web monitoring package as well for my 24/7/365 Operations desk to be able to actively "Green Board" monitor the network.