http://purl.org/tcl/home/man/tcl8.4/TkCmd/messageBox.htm
Note: Unlike most Tk widgets, this widget's name (messageBox) is not the same as its creation command (tk_messageBox).
incr Widgets also has a messageBox - once their DNS problems are resolved, a URL to their doc will be inserted in place of this part of the msg.
With recent versions, the Windows messageBox is "native", while the Unix one continues to rely on the implementation visible in ./library/msgbox.tcl. Before version 8.something, both Win* and Unix used msgbox.tcl (what about Mac?). Notice that this means that, if there's something about messageBox you don't like, you have the source code readily available for your enhancements.
Tk syntax help - Arts and crafts of Tcl-Tk programming
- [Category Command]