tclock is a transparent clock for X11 that uses the shaped window extension. tclock is a fairly old piece of software, well before the recent deluge of toolkits, clocks, window managers, etc.
tclock currently requires an ANSI compiler and X11 libraries of a relatively recent vintage. tclock was originally written for X11R4, but it requires R5 now since the API changed slightly.
Note that tclock does not rely on GTK+, Qt, or even Xt. (Making it use Xt is probably a good idea, though.)
A working version exists and will be released, uh, real soon now.
New releases are announced on the tclock-announce mailing list. Subscribe here.
tclock is covered under the GNU GPL (version 1 or later).
tclock was originally written by Rob Mayoff, and as such, is Copyright 1991 Rob Mayoff.
Recent bugs, this web page, and possibly even maintenace are due to Tim Showalter.
There, on the upper-right of this window. Actual size. A lot of things are configurable, either too many or not enough depending on your point of view (personally, I turn the tick marks off, but if I did that for this page it wouldn't look like a clock).