Comp.lang.tcl questioners frequently demand some sort of gadget to monitor-an-ongoing-process-and-display-the-result-in-a-text. Tailing widget is the simplest answer CL knows that meets the common intent of these requests.
There are lots of variations on this theme, though. tailf carefully implements the functionality of the Unix tail(1), and adds a bit of filtering capability. I've written a few examples [L1 ] that show how different combinations of IPC, fileevent, after, ... can co-operate to monitor continuing processes.
A utility to filter the output of tail is greptail. It lazily uses the unix tail(1) program.
Also see directory notification package in Tcl (Linux 2.4 and higher).