jabber.el supports a subset of JEP-0050, the standard for Ad-Hoc Commands. As the name implies, this can be used for just about anything. However, at the time of this writing it seems that jabber.el is the only implementation.
Currently, ad-hoc commands are used for setting presence remotely. If you realize that you forgot to set your client to “away” with a low priority, you can do it remotely.1
The commands for executing ad-hoc commands are available under the Service menu, which is opened by typing C-c C-s.
To find which commands are available, run “Request command-list”
(jabber-ahc-get-list).2
To run a command from the list, put point over it and run “Execute
command” (jabber-ahc-execute-command), accepting the defaults
for JID and node. (If you already know those, you could of course
enter them yourself) The form you get should hopefully be
self-explanatory.