I wrote this thing forever ago, it's a writeup of all the basic commands and such. People can snag and c/p or w/e. It's not accurate for Rhost, but I believe it should be for both Mush and Mux.
There are two kinds of code you'll generally need to worry about on a game. They are softcode and hardcode. Hardcode is the code that is the program itself, with certain commands already built in. They are generally the same across all games, with minor variances due to there being a few different hardcode packages. These commands can typically be identified by a lack of a prefix to start them (ie: say instead of +say), or by that prefix being @. Softcode is the code that changes from game to game, and is generally the code that was created for whatever game you are on. There are exceptions -- some pieces of softcode that are freely available and widely used -- but that's typical. Usually, these are all prefixed with a +.
Help commands:
+help -- accesses the index of softcode help files
+help <topic> -- accesses a help file for a specific softcode command
help -- access the index of hardocde help files
help <topic> -- accesses a help file for a specific hardcode command
Communication commands (I will use & for separating the commands and their aliases here):
say & " -- Says something.
input -- "Hello.
You see -- You say, "Hello."
The room sees -- Jobob says, "Hello."
Pose & : -- poses something
input -- :smiles.
The room sees -- Jobob smiles.
@emit & \\ -- emits something
Input -- \\Over there, Joebob sits on a chair.
The room sees -- Over there, Joebob sits on a chair.
ooc or sometimes +ooc, depending on game (softcode that often masquerades as hardcode in how it's coded) -- says something OOC
Input -- ooc Wow!
The room sees -- <OOC> Joebob says "Wow!"
(The output for this command varies, but that's the general gist.)
Page sends a message directly to another player / other players, regardless of what room they're in on the game. There's a variety of ways to use it, and the command for such is 'page' -- 'p' also works. You also may use : in it like you do with a pose to get the same effect.
p <name>=<message> -- standard page.
p <name> <name> <name>=<message> -- standard page to multiple people.
p <message> -- pages the last person(s) you paged.
p <name or names>=:<message>.
The output / what you see varies depending on whether you paged or paged a pose.
Channels are kind of difficult for me to explain in depth, but the basics are easy. If you want to try and puzzle it through yourself, the help files are - 'help comsys', and then 'help comsys commands'. Hardcode. 
The basics are:
@clist - pulls up all of the channels that are public that you can see. There are other hidden ones that will often be available to you, but you'd have to ask your staffer what they are.
addcom <alias>=<Channelname>
The channel name is case sensitive.
addcom rum=Rumors
This will add the alias for the channel and put you on the channel. That alias (rum) is what you type to talk on it -- rum La la la would then come across to everyone on the Rumors channel as:
[Rumors] Joebob says "La la la."
Then, to turn it on or off, it's
rum on
rum off
Bboards! This is one of those softcode packages that's widely used on the bulk of the games out there. Tis a bulletin board; most games store lots of information there, and a great deal of conversation usually happens on them.
+bbread <#>
Lists all of the posts on board #. The # is the bboard number (on the left-ish) that you see when you hit +bbread by itself.
+bbread <#>/<#>
Reads a post. The first # should be the bboard number, the second should be the actual post number on that bboard.
+bbread <#>/U
Reads all unread posts on bboard #. Be prepared for spam.
+bbpost <#>/<subject>
This starts a bboard post on board# -- commands of how to write the text and actually post it will helpfully pop up when you do this.
Mail. Like e-mail, only on the Mu*. >.>
@mail -- checks to see if you have any.
@mail/read <#> -- reads said message.
@mail <name>=<subject> -- starts a mail message.
-<text> -- adds text to mail message.
@mail/send -- sends a message.
Miscellaneous commands:
+who & WHO
Sees who's on. WHO (case sensitive) will show a listing of players, their time connected, their idle time, and their @doing, which is typically just some random comment, the maximum characters of which is not very big. +Who is the softcode version, and will generally show the player name and then some other field(s), depending on what the game prioritizes.
@doing <blah>
Sets your doing.
+where
Shows the location of all non-unfindable players. This is not on all games, but it is on a lot of games.
