Here's the best scanner of the www, and i could write down all his features but i've already said the main two, wich are:

network exploration;
security auditing;
tcp and udp port scanner;
operative system and application enumeration (version, name, etc...);

It also as the abilitie to scan large networks or single hosts, servers, whatever... .

Well, there's many other cool and usefull functions in NMap, but visit the site:

www.insecure.org

And there download the version for your operative system.

Notes:

In windows versions, the nmap can work in a ms-dos and/or GUI mode, so, download the one you like the most.

Also, if you're thinking that this scanner doesn't have more functions then the usuall free scanners, think again!

I've been curious about these kinds of tools for a while now, but never found one that I could use very easily. Would this utility happen to include a packet sniffer? Sometimes I'd like to be able to analyze the traffic that's going through my network card, especially if I have reason to think I've been rooted or some other spyware nastiness.

Thanks!

This is not exactly a sniffer or network analyzer (original name for sniffer), and i don't know what kind of network you want to analyze.

Is it a wired or wireless network?

Hi lyon2, it's a wired network. Thanks.

Well, there's tons of them, but if really want to see the best security tools, including sniffers and others very usefull, visit this link (list of the top 75 security tools) :

http://www.insecure.org/tools2000.html