You can use the msg
tool:
Send a message to a user.
MSG {username | sessionname | sessionid | @filename | *}
[/SERVER:servername] [/TIME:seconds] [/V] [/W] [message]
username Identifies the specified username.
sessionname The name of the session.
sessionid The ID of the session.
@filename Identifies a file containing a list of usernames,
sessionnames, and sessionids to send the message to.
* Send message to all sessions on specified server.
/SERVER:servername server to contact (default is current).
/TIME:seconds Time delay to wait for receiver to acknowledge msg.
/V Display information about actions being performed.
/W Wait for response from user, useful with /V.
message Message to send. If none specified, prompts for it
or reads from stdin.
The call
msg * Some text
doesn't block. It also has the nice capability of closing the message box again after a set amount of time if required.
On a side note, though, you shouldn't really use those things. Monologs (like message boxes with exactly an OK button) have an information efficiency of 0 % (cf. Jef Raskin: The Humane Interface. Section 4–3: Measurement of Interface Efficiency or Aza Raskin: Monolog Boxes and Transparent Messages or Aza Raskin: Know When to Stop Designing, Quantitatively).