Tool: nc (Netcat)
Use: Enumeration and much more.
Version: 1.10-41+b1
OS: Windows, Linux, MacOSX & others.
Vendor/Author: The Hobbit <[email protected]>
URL: Unknown
Status: Unknown/Inactive
Description: TCP/IP swiss army knife

From the MAN page:
netcat is a simple unix utility which reads and writes data across network connections, using TCP or UDP protocol. It is designed to be a reliable "back-end" tool that can be used directly or easily driven by other programs and scripts. At the same time, it is a feature-rich network debugging and exploration tool, since it can create almost any kind of connection you would need and has several interesting built-in capabilities. Netcat, or "nc" as the actual program is named, should have been supplied long ago as another one of those cryptic but standard Unix tools.

In the simplest usage, "nc host port" creates a TCP connection to the given port on the given target host. Your standard input is then sent to the host, and anything that comes back across the connection is sent to your standard output. This continues indefinitely, until the network side of the connection shuts down. Note that this behavior is different from most other applications which shut everything down and exit after an end-of-file on the standard input.

Netcat can also function as a server, by listening for inbound connections on arbitrary ports and then doing the same reading and writing. With minor limitations, netcat doesn't really care if it runs in "client" or "server" mode -- it still shovels data back and forth until there isn't any more left. In either mode, shutdown can be forced after a configurable time of inactivity on the network side.

And it can do this via UDP too, so netcat is possibly the "udp telnet-like" application you always wanted for testing your UDP-mode servers. UDP, as the "U" implies, gives less reliable data transmission than TCP connections and some systems may have trouble sending large amounts of data that way, but it's still a useful capability to have.
This tool really is what it claims - a swiss army knife. Even though the tool seems simple, it is capable of doing various things such as transfer files, send a shell across the network, work as a port scanner, act as a server and a client and much more. This tool is often used to verify if a service is up, and if it is, Netcat will allow you to do banner grapping.
The tool is very stable and works well with a script around it. It even has the capability to work as a backdoor.