Nessus is free. Right? Sure, you can go right there and download it https://www.nessus.org
Unfortunately, if you want to DO anything with it, you need a console. There are a couple of free ones, but the only enterprise ready console is called Lightning (soon to be Security Center) from Tenable. For my business, what I need is automated scanning and serious reporting features.
Lightning is http://www.tenablesecurity.com/products/lightning.shtml
The free client is http://nessuswx.nessus.org/
And yes, naturally, they license Lightning by active IP addresses, so it grows expensive REALLY fast. Maybe I’m missing the boat here. Once again, though, there’s no such thing as free software 🙂 And the linux part isn’t the problem–it’s the licensing plan. Once you get to $20K or so, you should be at an enterprise level and be done with it.