Slash commands are entered in the chat window preceeded by a slash. Although many of these are covered in the game's manual, there are also quite a few which are not documented anywhere but by the /cmdlist command. As these commands are all quite useful when making macros and keybinds, knowing which ones are available can help you know what is possible with your character.
Since there are so many slash commands, I am going to divide them into categories:
These commands control the player's character, and allow him to move, select targets, and use his powers. These are probably the most valuable commands when setting up macros and keybinds.
These commands control the user interface, and allow the user to change the way the screen is laid out to fit his personal style. The commands to actually set up and load keybinds are also in this category.
These commands control the chat interface, and help the user communicate with other players. I will also include the commands for user feedback to the developers.
These commands are used to set up and manage teams. They include commands for searching for teammates. I will also include the commands for setting up Arena battles.
These commands are specifically used for managing supergroups and coalitions.
These commands have been defined specifically for Masterminds to control their pets. They do not work with any other type of pets, although any archetype can use the /release_pets and /petselect commands. (So they are also listed in the Character Commands above)
Stances are "aggressive", "defensive" and "passive". Actions are "attack", "goto", "follow", "stay" and "dismiss". Commands that require a pet name, stance, action, or power name will take any unique portion of the name, thus you could use "at" instead of "attack". If you use a portion of a pet name that is not unique (for instance, "zombie" if your zombies are named "zombie 1" and "zombie 2") it will command both of them. If you name your pets, however, you can use the /petcom_pow command to command all those of a given type.
There are a number of emotes that have been defined for City of Heroes, most of which are not documented in the manual. Note that if you type anything as an emote that is not in this list (for instance, /emote thinks) you will see "thinks" in a thought balloon, and in the chat window as "So-and-so thinks". In this way you can emote just about anything.
Case does not matter, so you can type /emote yes or /emote Yes. Many emotes are continuous, in order to cancel them, just move. Note that afk and newspaper are also commands. As an emote, afk simply is the same as the newspaper, it does not make you go afk. The /newspaper command, however, is completely unrelated to its emote.
These commands change the graphics capabilities of the game. This commands should not normally be used, as they are primarily intended for debugging, and can make the game impossible to play, or have other side effects. Thus, I'm putting them in a section of their own.