ATelnet is a MUD and telnet client that runs inside the mIRC IRC-client. It opens a child-window inside mIRC, like the channel and query windows, and can therefore merge with mIRC's features and interface while still providing several tools of its own for use with MUDs. It is free software under the GNU General Public License.
It should also be emphasized that this is primarily a MUD client, not a telnet program. It does not support the set of features required to access interactive shells properly via telnet. There are far better places to implement such things than some crappy IRC client with an idiotic scripting language. (Indeed, this is the only mIRC-script I've ever written, or will ever write. mIRC's strength lies entirely in its GUI, everything I've seen under the hood is just plain... Well, let's keep this civil, shall we.)
ATelnet is no longer developed or supported. With the release of mIRC 6.x, the author of mIRC destroyed script compatibility again, so ATelnet only works on the mIRC 5.9x series. Anyhow, nowadays there are many better IRC clients (like "irssi") available, so I recommend you use those instead of mIRC, which has sadly become (in my opinion) just a crappy piece of hideous bloatware, while most of its old flaws still remain (or have become worse). For a better MUD client than ATelnet, try "tinyfugue".
Personally, I have stopped using Windows entirely, since I believe there is more and better software available for my needs on *nix systems (Linux, OpenBSD, etc). Hence, I am unable to run mIRC and therefore ATelnet. So please don't ask me anything about ATelnet anymore, I am unable and unwilling to tinker with it now.
However, there's no reason why it wouldn't be as useful now as it was back when I made it, for those that are able to use it - just don't swith to a new, bloated and incompatible, version of mIRC... Thanks to all ATelnet users and have fun.