Nmap ("Network Mapper") is a utility for network exploration, administration, and security auditing. It uses IP packets in novel ways to determine which hosts are available online (host discovery), which TCP/UDP ports are open (port scanning), and what applications and services are listening on each port (version detection). It can also identify remote host OS and device types via TCP/IP fingerprinting. Nmap offers flexible target and port specifications, decoy/stealth scanning for firewall and IDS evasion, and highly optimized timing algorithms for fast scanning.
Tool currently broken
Bidwatcher is an excellent tool when it works, but they have not made an updated release since eBay changed their web page and broke Bidwatcher's bidder more than 3 months ago. The latest CVS version may (or may not) fix the problem. The CVS head didn't compile for me, but I didn't spend much time messing with it.
If you haven't used bidwatcher in a few days or more, always test a real-time bid before relying on the snipe function for anything important. Because if a snipe fails (due to eBay changing their html submission process), you probably won't have enough time to bid manually.
Re: The coolest tool for Linux!
Thanks, I am glad you find Nmap useful! I agree that an explicit field for command-line arguments would be valuable. But for now, sticking them in the "Target(s)" field works fine. Newer versions of NmapFE also support a larger selection of Nmap arguments, making this less-often necessary. And of course you can always use the standard command-line version of Nmap for custom commands.