I find nmap a great tool to find out what is realyl on my network. The new service identification tool, which is unfortunately slow, actually connects to each open port and attempts to identify the service running. It is usually able to do this with great accuracy.
The ability to transform the xml report to a nice html report via a provides xslt file is quite nice for situations where a full nessus run is not necessary or practical.
The windows version runs very well these days. In years past it was only available via a third party fory. However, these days Fydor supports it in full.
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The tool is quite good. Its main use is for converting from one format to another, but this is the netcat of digital A/V. Anything you could imagine doing to audio and video with a command line tool can be done with this tool.
The developers are of the mindset that there is no need for formal releases and that with all the formal testing in place and given SVN revision will work. For the most part this is true. Unfortunatly, there are a myraid of command line options, and at any given moment a given combination of them will fail.
The interface is quite spartan, but it manages to work well. I was a fan of GAIM, now pidgin, but on windows this is my IM client of choice.
The one feature it lacks is tabbed chat windows.
I've been using this tool for a number of years, both standalone and with the windows version control clients TortioseSVN and TortioseCVS. It just simply works and it keeps getting better. The ability to recursively diff directory's is very useful when manually merging two branches of a source tree.