anyone knows what features it offers beyond what iftop already offers (being web based doesn't count)? thanks
Isn't iftop for a single interface? This can be a netflow(s-flow,etc) collector and thus give data about an intersection of links. This gives you the chance to add cpu and memory overhead to your switches and routers and also sampled data to draw dubious conclusions from! (Sorry, bitter ex net-eng. Modern data center speed and data volume are no place for sampling IMO. ASIC buffer vs microburst = hard to find badness.)
Most modern routers do sflow collection in hardware. There's no CPU or memory overhead for having it on.
They're completely different tools with different purposes. Ntop is a tool you run on your network to introspect network flows between various hosts. The iftop utility provides a look in to what is flowing on an interface on your local machine. With ntop, you can aggregate collectors from many locations in order to develop a detailed understanding of how data flows in and out of your network.
Windows and OSX support, although it seems that if you don't compile the Windows version yourself, you're limited to 2000 packets[1] or to pay[2].
You can also persist data either into memory for the duration of your uptime of the ntop process or ultimately into a sql store. This makes longer term investigation possible.