I've always loved InterMapper. It's one of the best Mac tools for actively monitoring network utilization and just about any other SNMP info.
My initial purchase years ago for an unlimited license was quite reasonable. As time passed, the cost of keeping the product current increased out of my budget. That's not to say the product and its developer isn't worth it.
Forced to look for alternatives, I found The Dude from MikroTik. I dabbled with it on its native Windows occasionally, finding it to be quite complex, but full of potential. Recently, I used WineBottler to install it as an app within OS X and was surprised to find it working quite well (2.8 Quad Xeon with the Dude server running on a separate Win box).
The Dude doesn't have the beloved marching ants of InterMapper, but it can gradually change the link colors as utilization grows. I still have much to learn, but with some effort, I imagine it's possible to do many of the things that IM once did for me. Not bad for free. Of course, you have to ask yourself, how much time do you have?